Optical disc unit

ABSTRACT

An optical disc unit  2001  comprises: reflective surface detection means  1010  for detecting a reflective surface; focus control means ( 1202, 1003, 1009, 1008, 1003, 1012, 1005  and  1204 ) for performing focus control to a reflective surface so that the distance between the focal point of an optical beam applied to an optical disc  2100  and the reflective surface is within a predetermined error limit; shift means  1007  for shifting the focal point of the optical beam in a direction perpendicular to the optical disc; and control means  1006  for controlling the focus control means and the shift means.

TECHNICAL FIELD

[0001] The present invention relates to an optical disc unit forrecording information such as digital video information on an opticaldisc in high density and reproducing information recorded on an opticaldisc, and in particular, an optical disc unit which is capable ofperforming an accurate focus control to an information surface of anoptical disc.

BACKGROUND ART

[0002] Recently, optical discs have been drawing attention asexchangeable media with a high capacity and an optical disc unit isexpected to be used more widely as a video recorder in the future. Oneof the reasons why the optical disc is a high-capacity exchangeablemedium is that it allows performance of recording/reproduction withoutcontacting the medium. In other words, a laser beam is focused and aninformation recording layer of an optical disc is irradiated with thefocal point thereof, and thus recording and reproduction are performed.Therefore, even when some dirt or dust is adhered on the surface of thedisc, a head crash does not occur as it does, for example, in magneticrecording.

[0003] However, such a characteristic of the optical disc is based on amajor premise: a focus control, i.e., a control for maintaining thedistance between the focal point of a convergence laser beam and theinformation recording layer within an allowable error limit is stablyperformed. Once the focus is out of control, an objective lens actuatorof an optical head runs out of control and may crash into the opticaldisc surface. Such an off-focus frequently occurs particularly whenfocusing is performed, i.e., during the process of shifting a laser beamfocal point into a capture range of the focus control and then closing afocus control loop, immediately after an optical disc drive isactivated. Thus, conventionally, various methods of focusing have beenstudied.

[0004] For example, in a method described in Japanese Laid-OpenPublication No. 9-115147, whether an initial laser beam focal point isclose or far relative to the optical disc information recording layer isdetermined beforehand and the focusing process is performed inaccordance with this initial state. When the focal point is determinedto be close relative to the information recording layer, the objectivelens actuator is driven to bring the focal point closer to theinformation recording layer and when the focal point enters the capturerange of the focusing, the operation is switched to a close loopoperation of the focus control. Alternately, when it is determined to befar, the actuator is driven to be further, and then the operation issimilarly switched to the close loop operation.

[0005] However, the conventional method has the problem of an increasedrate of collision between an objective lens and the optical disc surfacein the case where a working distance of the objective lens (the distancebetween the objective lens and the optical disc surface) is smaller thanthe wobbling of the optical disc. Specifically, there is no problem whenthe focusing is achieved with one attempt. However, if it fails, eventhough the objective lens actuator does not run out of control, in thecase where the wobbling of the disc is greater than the workingdistance, there is a possibility that the disc collides into theobjective lens.

[0006] For the currently available CD players and DVD players, the NA(numerical aperture) of the objective lens is about 0.45 to 0.6 at most.Thus, it is possible to ensure a working distance of 0.5 mm or more.Therefore, wobbling of about 0.2 mm, which may usually occur, can besufficiently absorbed. However, if the NA is raised to its limit inorder to further increase the recording capacity in the future, thedesigned working distance of the objective lens becomes extremely small.For example, if the NA is 0.85, even if the thickness of a protectivelayer is lowered to about 0.1 mm (the thickness of a protective layer is1.2 mm in a CD and 0.6 mm in a DVD), the working distance is about 0.15mm.

[0007] The present invention was conceived in view of such problems. Theobjective of the present invention is to provide an optical disc unitfor performing a focus control which is capable of significantlyreducing the frequency of collisions of an objective lens into anoptical disc surface even when a sufficient working distance of theobjective lens is not ensured due to an increased capacity of theoptical disc.

[0008] As a conventional optical disc unit, there is an optical discunit in which an optical beam generated from a light source such as asemiconductor laser is focused on and applied to an optical discrotating with a predetermined rotation number and signals recorded onthe optical disc are reproduced. The optical disc includes a pluralityof tracks formed in spiral forms. The tracks are formed of grooveshaving concave and convex portions. A recording film of a phase-shiftmaterial or the like is attached to an information surface. Forrecording information on the optical disc, the strength of the opticalbeam is changed in accordance with the information, with a focus controlperformed such that a focal point of the optical beam is on theinformation surface, and with a tracking control performed such that thefocal point is on the tracks. Thus, the reflectance of the recordingfilm is varied. For reproducing information on the optical disc, lightreflected off the optical disc is received at a photodetector,similarly, with the focus control performed such that a focal point ofthe optical beam is on the information surface and with the trackingcontrol performed such that the focal point is on the tracks. The outputof the photodetector is processed to reproduce the information.

[0009] A focus error signal indicating misalignment between aninformation surface of an optical disc and a focal point of an opticalbeam is detected by astigmatic method or the like. The focus errorsignal becomes zero when the focal point is on the information surface.In general, the detection limit of the astigmatic method is about 10 μm.Thus, for operating a focus control system, the objective lens is movedbeforehand so as to shift the position of the focal point into thedetection limit. The focus control is performed at the time when thefocus error signal crosses zero. However, when the focal point passesthe optical disc surface, the focus error signal also crosses zero. Ifthe focus control is performed when zero-crossing occurs at the opticaldisc surface, the focal control is performed such that the focal pointis positioned on the optical disc surface. In order to prevent this, thefact that the reflectance of the information surface is higher than thatof the optical disc surface is utilized. More specifically,zero-crossing which occurs at the recording surface is detected when thelevel of the total internal reflection amount is detected to exceed apredetermined level.

[0010] Recently, a rewritable optical disc which has two informationsurfaces on one side of the optical disc has been proposed. Hereinafter,such an optical disc is referred to as a doublelayer optical disc. Inthe doublelayer optical disc, when information on the informationsurface further from the objective lens is reproduced, it is reproducedwith an optical beam which has been transmitted through the informationsurface closer to the objective lens. Thus, the doublelayer optical discis designed such that the reflectance of the information surface closerto the objective lens is low.

[0011] Accordingly, the amount of light from each of the informationsurfaces which is received at a photodetector becomes small. The opticaldisc having one information surface is referred to as a single-layeroptical disc.

[0012] As described above, the level of the total internal reflectionamount at each of the information surfaces becomes low in thedoublelayer optical disc. Thus, it is difficult to distinguish theoptical disc surface and the information surfaces based on the totalinternal reflection amount. This means that it is difficult to ensurethe focusing to the information surface if the focusing is performed ina method similar to the conventional method.

[0013] The present invention is conceived in view of the above-describedproblem. The objective of the present invention is to provide an opticaldisc unit which is capable to ensure the focusing to the informationsurface even in the case where difference between the amount of thetotal internal reflection off the optical disc surface and the amount ofthe total internal reflection off the information surface is small (forexample, in the case where a doublelayer optical disc is used).

DISCLOSURE OF THE INVENTION

[0014] The present invention provides an optical disc unit for anoptical disc having one or more information recording layers and one ormore protective layers formed on the information recording layers,comprising: reflective surface detection means for detecting areflective surface; focus control means for performing focus control tothe reflective surface such that a distance between a focal point of anoptical beam applied to the optical disc and the reflective surface iswithin a predetermined error limit; shift means for shifting theposition of the focal point in a direction perpendicular to the opticaldisc; and control means for controlling the focus control means and theshift means, wherein the control means controls the shift means suchthat the focal point of the optical beam shifts toward the protectivelayer until a surface of the protective layer is detected by thereflective surface detection means, the control means controls the focuscontrol means to perform focus control to the surface of the protectivelayer when the surface of the protective layer is detected, the controlmeans controls the shift means to release the focus control to thesurface of the protective layer and shifts the focal point of theoptical beam toward the information recording layer until a surface ofthe information recording layer is detected by the reflective surfacedetection means, and the control means controls the focus control meansto perform focus control to the surface of the information recordinglayer when the surface of the information recording layer is detected,thereby achieving the above-described objectives.

[0015] A feedback gain of the focus control to the surface of theprotective layer and a feedback gain of the focus control to the surfaceof the information recording layer may be set such that the product ofthe feedback gain of the focus control to the surface of the protectivelayer and a reflectance of the surface of the protective layer is equalto the product of the feedback gain of the focus control to the surfaceof the information recording layer and a reflectance of the surface ofthe information recording layer.

[0016] Information indicating the reflectance of the informationrecording layer may be formed beforehand on the surface of theprotective layer, the control means may read the information from thesurface of the protective layer while the focus control to the surfaceof the protective layer is performed, and may set the feed back gain ofthe focus control to the surface of the information recording layerbased on the information.

[0017] The reflectance of the surface of the protective layer may be 3%to 5%.

[0018] The present invention provides an optical disc unit for anoptical disc having one or more information surfaces having a pluralityof tracks formed thereon, comprising: tracking error detection means fordetecting a misalignment between an optical beam applied to the opticaldisc and one of the plurality of the tracks corresponding thereto, andoutputting a tracking error signal indicating the misalignment;amplification detection means for detecting amplification of thetracking error signal; focus control means for performing focus controlsuch that a distance between a focal point of the optical beam and theinformation surface is within a predetermined error limit; shift meansfor shifting the position of the focal point of the optical beam towardthe optical disc; and control means for controlling the focus controlmeans and the shift control means, wherein the control means controlsthe shift means such that the focal point of the optical beam is shiftedin a direction traversing tracks formed on the information surface ofthe optical disc and approaches the optical disc with an operation ofthe focus control means stopped; and the control means allows the focuscontrol means to start the operation only when the amplification of thetracking error signal is detected to become a predetermined value orhigher by the amplification detection means, thereby achieving theabove-described objectives.

[0019] Each of the plurality of the tracks formed on the informationsurface may be wavy.

[0020] An optical disc unit may further comprises zero-cross detectionmeans for detecting that a focus error signal indicating a misalignmentbetween the focal point of the optical beam and the information surfacecrosses zero, and the control means may start an operation of the focuscontrol means when the amplification of the tracking error signal isdetected to become the predetermined value or higher by theamplification detection means and the focus error signal is detected tocross zero by the zero-cross detection means.

[0021] An optical disc unit may further comprise a band-pass filter, andthe tracking error signal may be supplied to the amplification detectionmeans via the band-pass filter.

[0022] The control means may control rotations of the optical disc suchthat the number of rotations of the optical disc when the amplificationof the tracking error signal is detected by the amplification detectionmeans is smaller than the number of rotations of the optical disc wheninformation recorded on the information surface of the optical disc isbeing reproduced.

[0023] The control means may control strength of the optical beam suchthat strength of the optical beam when the amplification of the trackingerror signal is detected by the amplification detection means is smallerthan strength of the optical beam when information recorded on theinformation surf ace of the optical disc is being reproduced.

[0024] The control means may perform the focus control with rotations ofthe optical disc stopped and controls the rotations of the optical discsuch that the optical disc starts to rotate after the distance betweenthe focal point of the optical beam and the information surface isdetected to be within the predetermined error limit.

[0025] The present invention provides an optical disc unit for anoptical disc having one or more information surfaces, comprising: focuserror detection means for outputting a focus error signal indicating amisalignment between a focal point of an optical beam applied to theoptical disc and a predetermined surface; shift means for shifting theposition of the focal point of the optical beam in a directionperpendicular to the optical disc; focus control means for performingfocus control to the predetermined surface such that a distance betweenthe focal point of the optical beam and the predetermined surface iswithin a predetermined error limit by controlling the shift means basedon the focus error signal; zero-cross detection means for detecting thatthe focus error signal crosses zero; and control means for controllingthe focus control means and the shift means, wherein the control meanscontrols the shift means such that the focal point of the optical beamshifts in a first direction toward a surface of the optical disc untilthe focus error signal is detected to cross zero for the first time bythe zero-cross detection means, the control means controls the shiftmeans such that, when the focus error signal is detected to cross zerofor the first time, the focal point of the optical beam further shiftsin the first direction by a predetermined distance which is larger thana distance between the surface of the optical disc and the informationsurface, the control means controls the shift means such that, until thefocal point of the optical beam has been further shifted in the firstdirection by the predetermined distance and when the focus error signalis detected to cross zero for the second time by the zero-crossdetection means, the focal point of the optical beam is shifted towardthe information surface in a second direction opposite to the firstdirection, and the control means controls the focus control means toperform the focus control to the information surface when the focuserror signal is detected to cross zero for the second time, therebyachieving above-described objectives.

[0026] The control means may perform the focus control with rotations ofthe optical disc stopped and controls the rotations of the optical discsuch that the optical disc starts to rotate after the distance betweenthe focal point of the optical beam and the information surface isdetected to be within the predetermined error limit.

[0027] The present invention provides an optical disc unit for anoptical disc having one or more information surfaces, comprising: focuserror detection means for outputting a focus error signal indicating amisalignment between a focal point of an optical beam applied to theoptical disc and a predetermined surface; shift means for shifting theposition of the focal point of the optical beam in a directionperpendicular to the optical disc; focus control means for performingfocus control to the predetermined surface such that the distancebetween the focal point of the optical beam and the predeterminedsurface is within a predetermined error limit by controlling the shiftmeans based on the focus error signal; zero-cross detection means fordetecting that the focus error signal crosses zero; and control meansfor controlling the focus control means and the shift means, wherein thecontrol means controls the shift means such that the focal point of theoptical beam shifts toward the surface of the optical disc until thefocus error signal is detected to cross zero for the first time by thezero-cross detection means, the control means controls the focus controlmeans to perform focus control to the surface of the optical disc whenthe focus error signal is detected to cross zero for the first time, thecontrol means stores displacement information indicating displacement ofthe shift means in accordance with a rotation angle of the optical discin storage means while the focus control to the surface of the opticaldisc is performed, the control means controls the shift means such thatthe focal point of the optical beam shifts toward the informationsurface based on the displacement information stored in the storagemeans with an operation of the focus control means stopped until thefocus error signal is detected to cross zero for the second time by thezero-cross detection means, and the control means controls the focuscontrol means to perform the focus control to the information surfacewhen the focus error signal is detected to cross zero for the secondtime, thereby achieving above-described objectives.

[0028] The focus control means may control phase compensation such thata band in which a phase leads is wider, compared to when informationrecorded on the optical disc is being reproduced, for a predeterminedperiod after the focus control means has started the operation.

[0029] The focus control means may set a gain such that the gain issmaller, compared to when information recorded on the optical disc isbeing reproduced, for a predetermined period after the focus controlmeans has started the operation.

[0030] The present invention provides an optical disc unit for anoptical disc having a plurality of information surfaces, comprising:photodetection means for detecting light reflected off the optical discwhen an optical beam is applied to a predetermined surface among theplurality of information surfaces; focus error detection means foroutputting a focus error signal indicating a misalignment between afocal point of the optical beam and the predetermined informationsurface based on an output from the photodetection means; total internalreflection amount detection means for detecting an amount of totalinternal reflection off the optical disc based on the output from thephotodetection means; and normalization means for generating anormalized focus error signal by dividing the focus error signal by avalue obtained by subtracting a signal value corresponding to areflection amount reflected off information surfaces other than thepredetermined information surface of the optical disc from the output ofthe total internal reflection amount detection means, thereby achievingthe above-described objectives.

[0031] An optical disc unit may further comprises: shift means forshifting the position of the focal point of the optical beam in adirection perpendicular to the optical disc; focus control means forperforming focus control such that a distance between the focal point ofthe optical beam and the predetermined information surface is within apredetermined error limit by controlling the shift means based on thenormalized focus error signal; and focus gain measurement means formeasuring a gain of a system of the focus control, and the signal valuemay vary depending on an output from the focus gain measurement means.

[0032] An optical disc unit may further comprises shift means forshifting the position of the focal point of the optical beam in adirection perpendicular to the optical disc, and the signal value mayvary such that amplification of the normalized focus error signal is aconstant value when the shift means is driven such that the focal pointof the optical beam passes through the predetermined information surfaceof the optical disc.

[0033] The signal value may vary depending on each of the plurality ofthe information surfaces.

[0034] An optical disc unit may further comprises stray light detectionmeans for detecting light reflected off information surfaces other thanthe predetermined information surface of the optical disc on which thefocal point of the optical beam is located, and the signal value mayvary based on an output from the stray light detection means.

[0035] An optical disc unit may further comprises: shift means forshifting the position of the focal point of the optical beam in adirection perpendicular to the optical disc; and control means forcontrolling the shift means based on the normalized focus error signalso as to control the shift means to shift the focal point of the opticalbeam to information surfaces other than the predetermined informationsurface of the optical disc.

[0036] The photodetection means may further include optical beamsplitting means for splitting light reflected off the optical disc intolight of an inner region near an optical axis and light of an outerregion far from the optical axis; the focus error detection means mayinclude inner focus error detection means for detecting a misalignmentbetween the focal point of the optical beam and the predeterminedinformation surface of the optical disc based on the light of the innerregion, and outer focus error detection means for detecting themisalignment between the focal point of the optical beam and thepredetermined information surface of the optical disc based on the lightof the outer region; and the control means may control the shift meansbased on at least one of an output from the inner focus error detectionmeans and an output from the outer focus error detection means so as tocontrol the shift means to shift the focal point of the optical means toinformation surfaces other than the predetermined information surface ofthe optical disc.

BRIEF DESCRIPTION OF THE DRAWINGS

[0037]FIG. 1 is a block diagram showing an exemplary structure of anoptical disc unit according to Embodiment 1 of the present invention;

[0038]FIG. 2A shows a change in the position of a focal point of aconvergence laser beam over sequences 1 through 4;

[0039]FIG. 2B shows a change in a focus error signal FE over thesequences 1 through 4;

[0040]FIG. 3 shows an example of an optical disc on which discinformation is formed on a surface of a protective layer;

[0041]FIG. 4 is a block diagram showing an exemplary structure of anoptical disc unit according to Embodiment 2 of the present invention;

[0042]FIG. 5 shows an example of a change in the FE signal;

[0043]FIG. 6 schematically shows tracks formed on the optical disc;

[0044]FIG. 7 shows a waveform of a TE signal when the optical beamtraverses the tracks;

[0045]FIG. 8 shows waveforms of the signals when the objective lensgradually approaches the information surface of the optical disc;

[0046]FIG. 9 shows waveforms of the signals when focusing is beingperformed;

[0047]FIG. 10 is a block diagram showing an exemplary structure of anoptical disc unit of Embodiment 3;

[0048]FIG. 11 shows a plurality of tracks formed on the informationsurface of the optical disc;

[0049]FIG. 12 shows waveforms of the signals when the optical beamtraverses the tracks with the focus control being performed;

[0050]FIG. 13 shows waveforms of the signals used in the optical discunit;

[0051]FIG. 14 is a block diagram showing an exemplary structure of theoptical disc unit according to Embodiment 4 of the present invention;

[0052]FIG. 15 shows waveforms of the signals used in the optical discunit;

[0053]FIG. 16 is a block diagram showing an exemplary structure of anoptical disc unit according to Embodiment 5 of the present invention;

[0054]FIG. 17 shows waveforms of the signals used in the optical discunit;

[0055]FIG. 18 shows an exemplary structure of an optical disc unitaccording to Embodiment 6 of the present invention;

[0056]FIG. 19 is a block diagram showing an exemplary structure of aphase compensation circuit;

[0057]FIG. 20 shows phase characteristics of the circuits included inthe phase compensation circuit;

[0058]FIG. 21 shows waveforms when the focusing is being performed;

[0059]FIG. 22 illustrates a working distance;

[0060]FIG. 23 is a block diagram showing an exemplary structure of anoptical disc unit according to Embodiment 7 of the present invention;

[0061]FIG. 24 illustrates a doublelayer optical disc 2187 and an opticalbeam 2106;

[0062]FIG. 25 shows waveforms of the signals used in the optical discunit;

[0063]FIG. 26 is a block diagram showing an exemplary structure of anoptical disc unit according to Embodiment 8 of the present invention;

[0064]FIG. 27 schematically shows a structure of a photodetector;

[0065]FIG. 28 is a block diagram showing an exemplary structure of anoptical disc unit according to Embodiment 9 of the present invention;

[0066]FIG. 29 shows waveforms of the signals used in the optical discunit;

[0067]FIG. 30 is a block diagram showing an exemplary structure of anoptical disc unit according to Embodiment 10 of the present invention;

[0068]FIG. 31 illustrates outer and inner focal points of the opticalbeam when the controlling FE signal is zero at the first informationsurface;

[0069]FIG. 32 shows waveforms of an outer FE signal and an inner FEsignal; and

[0070]FIG. 33 shows waveforms of the signals used in the optical discunit.

BEST MODE FOR CARRYING OUT THE INVENTION

[0071] Hereinafter, embodiments of the present invention will bedescribed with reference to the drawings.

[0072] (Embodiment 1)

[0073]FIG. 1 shows an exemplary structure of an optical disc unit 1001according to Embodiment 1 of the present invention.

[0074] The optical disc unit 1001 records information on an optical disc1100 and reproduces information recorded on the optical disc 1100. Theoptical disc 1100 has an information recording layer 1120 and aprotective layer 1110 formed on the information recording layer 1120.

[0075] The optical disc unit 1001 includes an optical head 1002 forirradiating the information recording layer 1120 with a convergencelaser beam.

[0076] The optical head 1002 includes a laser light source 1201, lightreceiving means 1202, an objective lens actuator 1204, and an objectivelens 1203.

[0077] The laser light source 1201 outputs a laser beam. The laser beamoutput from the laser light source 1201 is focused with the objectivelens 1203. As a result, the optical disc 1100 is irradiated with theconvergence laser beam. The convergence laser beam reflected off theoptical disc 1100 passes through the objective lens 1203 and is receivedby the light receiving means 1202. The objective lens 1203 is driven bythe objective actuator 1204.

[0078] The light receiving means 1202 is formed of, for example, lightreceiving sections divided into plural parts. A group of signals outputfrom the light receiving means 1202 (DOUT) are supplied to focus errorcalculation means 1003 and information reading means 1011. The focuserror calculation means 1003 generates a focus error signal (FE) fromthe signal group DOUT. The focus error signal varies in accordance withthe distance between the focal point of the convergence laser beam and areflective surface (a surface of the protective layer 1110 or theinformation recording layer 1120, which will be described in detaillater). The focus error signal FE feeds back to the objective lensactuator 1204 of the optical head 1002 via a gain amplifier 1009, aswitch 1008, adding means 1004, low-frequency compensation means 1012,and an actuator driver 1005.

[0079] Thus, a focus control loop for performing focus servo by thelight receiving means 1202, the focus error calculation means 1003, thegain amplifier 1009, the switch 1008, the adding means 1004, thelow-frequency compensation means 1012, the actuator driver 1005, and theobjective lens actuator 1204 is formed. The switch 1008 is used foropening and closing the focus control loop. The adding means 1004 isused for adding the output signal of focal point shift means 1007, whichwill be described later, to the focus control loop.

[0080] The light receiving means 1202, the focus error calculation means1003, the gain amplifier 1009, the switch 1008, the adding means 1004,the low-frequency compensation means 1012, the actuator driver 1005, andthe objective lens actuator 1204 act as focus control means forperforming the focus control to the reflective surface such that thedistance between the focal point of the convergence laser beam and thereflective surface is within a predetermined error limit by driving theobjective lens actuator 1204 based on the focus error signal FE.

[0081] The focal point shift means 1007 forcibly shifts the focal pointof the convergence laser beam vertically with respect to the opticaldisc surface. The output signal of the focal point shift means 1007 isadded to the focus control loop by the adder 1004.

[0082] A sequencer 1006 controls the focal point shift means 1007 andthe focus control means. The sequencer 1006 controls the focus controlmeans by switching the switch 1008 between on and off. When the switch1008 is on, the focus control loop is closed, and thus the focus controlmeans operates. When the switch 1008 is off, the focus control loop isopen, and thus the operation of the focus control means stops.

[0083] The sequencer 1006 may be formed of, for example, microprocessorswith programs for generating sequences 1 through 4 which will bedescribed later incorporated therein.

[0084] Hereinafter, with reference to FIGS. 2A and 2B, a function of thesequencer 1006 will be explained. The sequencer 1006 sequentiallyperforms the following sequences 1 through 4. FIG. 2A shows the changein the position of the focal point of the convergence laser beam overthe sequences 1 through 4. FIG. 2B shows the change in the focus errorsignal FE over the sequences 1 through 4.

[0085] (Sequence 1)

[0086] The sequencer 1006 controls the focal point shift means 1007 suchthat the focal point of the convergence laser beam shifts toward theprotective layer 1110. Such a control is performed by, for example, thesequencer 1006 issuing command M to the focal point shift means 1007. Inresponse to the command M, a DC current is supplied to the objectivelens actuator 1204. Therefore, the objective lens 1203 moves at apredetermined speed in a direction perpendicular to the optical disc1100.

[0087] In sequence 1, the switch 1008 is turned off. Thus, the focuscontrol loop is open and the focus control means is stopped.

[0088] Sequence 1 ends when a reflective surface (i.e., a surface of theprotective layer 1110) is detected by reflective surface detection means1010.

[0089] The reflective surface can be detected by, for example, detectingthat the focus error signal FE exceeds a predetermined threshold value(V_(th)) (FIG. 2B). Such a detection can be performed by utilizing aspecific characteristic of the focus error signal that, when thedistance between the focal point of the convergence laser beam and thereflective surface is short (i.e., within a detectable range), a focuserror signal having an amplitude approximately proportional to a focuserror can be obtained, but when the distance is out of the detectablerange, a signal having such amplitude cannot be obtained (the so-calleds-shape characteristic).

[0090] (Sequence 2)

[0091] The sequencer 1006 controls the focus control means so as toperform the focus control to the surface of the protective layer 1110.This is performed by switching the switch 1008 from off to on. Thereflective surface detection means 1010 outputs an output pulse signal Pwhen reflective surface detection means 1010 detects that the focuserror signal FE exceeds the predetermined threshold value (V_(th)). Inresponse to an edge of the output pulse signal P, the sequencer 1006generates a loop-on-signal (LON) for closing the switch 1008. Thus, theswitch is turned on, and the focus control to the surface of theprotective layer 1110 is started.

[0092] Sequence 2 ends when the focus control to the surface of theprotective layer 1110 is stabilized. For example, after a predeterminedtime period has passed from the beginning of the focus control, there isno problem to regard that the focus control is stabilized. Thepredetermined time period is preferably about ten times (or more) of aresponse time period determined depending on the focus control band. Forexample, if the response time corresponding to the focus control band of10 kHz is 0.1 ms, the predetermined time period is preferably 1 ms(=0.1ms×10) or more.

[0093] (Sequence 3)

[0094] The sequencer 1006 releases the focus control to the surface ofthe protective layer 1110. This is performed by switching the switch1008 from on to off.

[0095] Then, the sequencer 1006 controls the focal shift means 1007 suchthat the focal point of the convergence laser beam shifts toward theinformation recording layer 1120. Such a control is performed by, forexample, the sequencer 1006 issuing command M to the focal point shiftmeans 1007. In response to the command M, a DC current is supplied tothe objective lens actuator 1204. Thus, the objective lens 1203 moves ata predetermined speed in a direction perpendicular to the optical disc1100.

[0096] Sequence 3 ends when a reflective surface (i.e., the surface ofthe information recording layer 1120) is detected by the reflectivesurface detection means 1010.

[0097] The reflective surface is detected by the same method as themethod described with respect to sequence 1.

[0098] (Sequence 4)

[0099] The sequencer 1006 controls the focus control means so as toperform the focus control to the surface of the information recordinglayer 1120. This is performed by switching the switch 1008 from off toon.

[0100] The sequence 4 ends when the focus control to the informationrecording layer 1120 is stabilized.

[0101] As described above, with the optical disc unit 1001 according tothe present invention, focusing having two steps is performed with thecontrol by the sequencer 1006. The first step of the focusing is thefocusing to the surface of the protective layer 1110. The second step ofthe focusing is the focusing to the surface of the information recordinglayer 1120. With such a focusing having two steps, a risk of theobjective lens 1203 colliding into the optical disc 1100 can besignificantly reduced. The reason will be described in detail below.

[0102] In sequences 1 and 2, the first step of the focusing is performedto the surface of the protective layer 1110 not to the surface of theinformation recording layer 1120. The focusing is performed to aposition which is distant from that in a conventional method by thethickness of the protective layer 1110. In other words, the workingdistance is extended by the thickness of the protective layer 1110.

[0103] For example, if the original working distance (i.e., the distancebetween the surface of the protective layer 1110 and the objective lens1203 when the focus is on the information recording layer 1120) is 150μm, the substantial working distance is 250 μm, which is the originalworking distance plus the thickness of the protective layer 1110, 100μm. Therefore, even if wobbling of about 200 μm is generated byrotations of the optical disc 1100, it is possible to avoid collision ofthe objective lens 1203 into the surface of the protective layer 1110due to a focusing failure in most cases.

[0104] In addition, in sequence 2, tracking control to the wobbling ofthe optical disc 1100 is performed. Thus, in sequences 3 and 4, theinfluence of the wobbling of the optical disc 1100 can be virtuallyignored. This is because the information recording layer 1120 and theprotective layer 1110 undergo the same wobbling.

[0105] In sequence 4, the relative speed of the information recordinglayer 1120 to which the focusing is going to be performed and theobjective lens 1203 is substantially zero. Thus, the optical disc 1100can be regarded to be substantially static (in the direction ofwobbling). In sequence 3, the focus control loop is blocked, but theoperation state of the actuator before blocking the focus control loopis kept almost as it is. As a result, in sequences 3 and 4, the focusingto the information recording layer 1120 can be performed almost surely.

[0106] As described above, according to Embodiment 1 of the presentinvention, even if the optical head having the objective lens of high NAis used, it is possible to avoid the collision of the objective lensinto the surface of the optical disc as much as possible by performingthe focus control to the surface of the protective layer 1110, and thenperforming the focus control to the information recording layer 1120.

[0107] Usually, the reflectance R1110 of the surface of the protectivelayer 1110 and the reflectance R1120 of the information recording layer1120 are different. The sequencer 1006 appropriately sets a gain using again amplifier 1009 in order to correct differences between thereflectance R1110 and the reflectance R1120.

[0108] A feedback gain G1110 when the focus control to the surface ofthe protective layer 1110 is performed in sequence 2 and a feedback gainG1120 when the focus control to the surface of the information recordinglayer 1120 is performed in sequence 4 are preferably set to meet formula(1).

R 1110×G 1110=R 1120×G 1120  (1)

[0109] G1110 and G1120 are preferably set such that the product of R1110and G1110 equals the product of R1120 and G1120.

[0110] Setting the feedback gains G1110 and G1120 to meet formula (1)enables a loop gain of the entire control system to be maintainedconstant. As a result, it is possible to perform a stable focus controlto either of the surface of the protective layer 1110 and the surface ofthe information recording layer 1120.

[0111] The reflectance R1110 of the surface of the protective layer 1110is uniquely determined by the refractive index of the protective layer1110. On the other hand, the reflectance R1120 of the surface of theinformation recording layer 1120 significantly varies depending on thematerial of the information recording layer 1120. For example, if thematerial of the protective layer 1110 is a polycarbonate resin, which iscommonly used, the reflectance of the surface of the protective layer1110 is in the range of about 3 to 5%. The reflectance R1120 of thesurface of the information recording layer 1120 is in the range of 5 to20% in the case of a recordable and erasable media (for example, a phasechange film), 20 to 50% in the case of a rewritable media (for example,a pigment type material), and 70 to 90% in the case of read-only media(for example, an aluminum reflective film). The reflectance R1120 of thesurface of the information recording layer 1120 significantly variesdepending on the material thereof. Thus, there is no guarantee that therelationship which meets formula (1) is established unless thereflectance R1120 of the surface of the information recording layer 1120is known at the time of the focusing. Thus, there may be the case wherethe focusing to the information recording layer 1120 cannot be performedstably.

[0112] In order to surely obtain the reflectance R1120 of the surface ofthe information recording layer 1120, for example, informationindicating the reflectance R1120 may be formed on the surface of theprotective layer 1110 of the optical disc 1100 beforehand so as to allowthe information indicating R1120 to be read from the surface of theprotective layer 1110 by using the information reading means 1011 duringsequence 2 (i.e., during the focusing to the surface of the protectivelayer 1110). In sequence 4, the sequencer 1006 sets a control gain basedon the reflectance R1120 using the gain amplifier 1009.

[0113]FIG. 3 shows an example of an optical disc on which discinformation 1112 is formed on the surface of the protective layer 1110.The information indicating the reflectance R1120 of the surface of theinformation recording layer 1120 is included in at least part of thedisc information 1112. The disc information 1112 may be a bar codedirectly printed on the surface of the protective layer 1110 or may be alabel with a bar code or the like printed which is to be attached. Theinformation reading means 1011 may have any structure as long as itcompares every addition signal of the signal group DOUT which is outputfrom the light receiving means 1202 with a predetermined threshold valueand converts the bar code into a binary value based on the comparisonresult for detection.

[0114] (Embodiment 2)

[0115]FIG. 4 shows an exemplary structure of the optical disc unit 2002according to Embodiment 2 of the present invention.

[0116] In the present embodiment, a photodetector 2113 and a TE signalgeneration circuit 2102, which will be described later, act as trackingerror detection means. The tracking error detection means detects amisalignment between an optical beam applied to an optical disc 2100having an information surface with a plurality of tracks formed thereonand one of the tracks which corresponds thereto, and outputs a trackingerror signal which indicates the misalignment.

[0117] The photodetector 2113, an FE signal generation circuit 2115, aphase compensation circuit 2116, a power amplifier 2118 and an actuator2104 act as focus control means. The focus control means performs afocus control such that a distance between a focal point of the opticalbeam and the information surface of the optical disc 2100 is within apredetermined error limit.

[0118] A microcomputer 2122 acts as control means for controlling thefocus control means and the actuator 2104 (shift means).

[0119] The optical disc 2100 is attached to a motor 2127 and rotateswith a predetermined number of rotations. The motor 2127 is controlledby a motor control circuit 2126. The number of rotations of the motor2127 is set by the microcomputer 2122.

[0120] The optical disc 2100 has an information surface with a pluralityof tracks formed thereon (not shown in FIG. 4, see FIGS. 6 and 22). Theplurality of tracks are formed in spiral forms with concave and convexportions. The optical disc 2100 may be a single-layer disc or may be amultilayer disc, including a doublelayer disc.

[0121] A laser 2109, a coupling lens 2108, a polarized light beamsplitter 2110, a ¼ waveplate 2107, a total internal reflection mirror2105, the photodetector 2113, and the actuator 2104 are attached to theoptical head 2114.

[0122] The laser 2109 is connected to a laser control circuit 2101. Thelaser control 2101 drives the laser 2109 so as to have the lightemitting power set by the microcomputer 2122. An optical beam 2106generated by the laser 2109 attached to the optical head 2114 iscollimated into parallel light by the coupling lens 2108, and passesthrough the polarized light beam splitter 2110 and the ¼ waveplate 2107.Then, the light is reflected off the total internal reflection mirror2105 and focused and applied onto the information surface of the opticaldisc 2100 by an objective lens 2103.

[0123] The light reflected off the information surface of the opticaldisc 2100 passes the objective lens 2103 and is reflected off the totalinternal reflection mirror 2105. Then, it passes through the ¼ waveplate2107, the polarized light beam splitter 2110, a detection lens 2111, anda cylindrical lens 2112 and incident in the photodetector 2113comprising four light receiving sections. The objective lens 2103 isattached to a movable portion of the actuator 2104. The actuator 2104which acts as both the focusing direction shift means and the trackingdirection shift means includes a focusing coil, a tracking coil, apermanent magnet for focusing, and a permanent magnet for tracking. Whena voltage is applied to the focusing coil of the actuator 2104 by usingthe power amplifier 2118, a current flows through the coil. The coilreceives a magnetic force from the permanent magnet for focusing.

[0124] Thus, the objective lens 2103 moves in a direction perpendicularto the information surface of the optical disc 2100 (an up-and-downdirection in the figure). The objective lens 2103 is controlled based onthe focus error signal which indicates a misalignment between the focalpoint of the optical beam and the information surface of the opticaldisc such that the focal point of the optical beam 2106 is always on theinformation surface of the optical disc 2100.

[0125] When a voltage is applied to the tracking coil by using a poweramplifier 2125, a current flows through the coil. The coil receives amagnetic force from the permanent magnet for tracking. Thus, theobjective lens 2103 moves in a radial direction of the optical disc 2100(a direction traversing the tracks on the optical disc 2100, aright-and-left direction in the figure).

[0126] The photodetector 2113 is formed of four light receivingsections. The light reflected off the optical disc and incident on thephotodetector 2113 is sent to the focus error signal generation circuit2115 (hereinafter, referred to as the FE signal generation circuit 2115)and a tracking error signal generation circuit 2102 (hereinafter,referred to as the TE signal generation circuit 2102). The FE signalgeneration circuit 2115 generates a focus error signal (hereinafter,referred to as the FE signal) which indicates a misalignment between thefocal point of the optical beam 2106 and the information surface of theoptical disc 2100.

[0127] The optical system shown in FIG. 4 has a structure whichimplements a detection scheme of the FE signal which is generallyreferred to as an astigmatism method. The FE signal is sent to the poweramplifier 2118 via a phase compensation circuit 2116 and a switch 2117.

[0128] A current flows to the focusing coil of the actuator 2104 by thepower amplifier 2118. The phase compensation circuit 2116 is a filterwhich forwards a phase for stabilizing the focus control system. Thus,the objective lens 2103 is driven in response to the FE signal and thefocal point of the optical beam 2106 is always on the informationsurface.

[0129] The switch 2117 switches between a connection of a terminal a anda terminal c, and a connection between a terminal b and the terminal cin accordance with a potential at a control terminal d. In the presentembodiment, when the potential at the control terminal d is high, theterminal c and the terminal a are connected. When the potential is low,the terminal c and the terminal b are connected. The FE signal is alsosent to a zero-cross detection circuit 2119. When the zero-crossdetection circuit 2119 detects that the FE signal crosses zero, itoutputs a pulse signal. Hereinafter, the pulse is referred to as azero-cross signal.

[0130] The optical system shown in FIG. 4 has a structure whichimplements a tracking error signal detection scheme which is generallyreferred to as a push-pull method. Hereinafter, the tracking errorsignal is referred to as the TE signal. The TE signal generation circuit2102 detects a misalignment between the optical beam 2106 focused andapplied onto the information surface of the optical disc 2100 with theplurality of tracks formed thereon and the tracks of the optical disc2100 by the push-pull method. The TE signal is sent to a comparator 2128via a band-pass filter 2120 (hereinafter, referred to as BPF 2120) andan amplification detection circuit 2121.

[0131] The output from the comparator 2128 is sent to the microcomputer2122. A ramp generation circuit 2123 generates a signal which varies ina constant rate (i.e., a ramp waveform). The time period for generatingthe ramp wave is set by the microcomputer 2122. The output from the rampgeneration circuit 2123 is sent to the power amplifier 2118 via theswitch 2117. The switch 2117 is switched by the microcomputer 2122. Asine wave generation circuit 2124 generates sine wave. The time periodfor generating the sine wave is set by the microcomputer 2122. Theoutput from the sine wave generation circuit 2124 is sent to the poweramplifier 2125.

[0132] Now, an operation of focusing is described. The microcomputer2122 sets the predetermined number of rotations to the motor controlcircuit 2126, and then sets the predetermined light-emitting power tothe laser control circuit 2101. The microcomputer 2122 makes thepotential at the control terminal d of the switch 2117 low to connectthe terminal c and the terminal b. At this time, the focus control isnot being performed. The ramp generation circuit 2123 is activated tooutput the ramp wave. The current according to the ramp wave flowsthrough the focusing coil by the power amplifier 2118.

[0133] The objective lens 2103 moves toward the optical disc 2100 (in anupper direction in the figure). At the same time, the microcomputer 2122activates the sine wave generation circuit 2124 and a sine current flowsthrough the tracking coil by the power amplifier 2125. The objectivelens 2103 wobbles in a sine wave form in a direction traversing thetracks.

[0134] As described above, the objective lens 2103 approaches theoptical disc 2100 with wobbling in the direction traversing the tracks.When the focal point of the optical beam 2106 approaches the informationsurface of the optical disc 2100 and begins to traverse the tracks, theTE signal from the TE signal generation circuit 2102 is in the sine-waveform. The TE signal is sent to the amplification detection circuit 2121via the BPF 2120. Amplification detection means, i.e., the amplificationdetection circuit 2121, measures the amplification of the TE signal withthe optical beam moving in a direction orthogonal to the tracks. BPF2120 removes noises. The passband of the BPF 2120 is the frequency ofthe TE signal. The frequency of the TE signal depends on the pitch anddecentration of the tracks and the number of rotations of the opticaldisc. In terms of a usual optical disc unit and an optical disc, itranges from tens Hz to several KHz.

[0135] The amplification of the TE signal is detected by theamplification detection circuit 2121. When the amplification of thedetected TE signal becomes a predetermined value or higher, the outputfrom the comparator 2128 becomes high and the focal point of the opticalbeam is detected to be near the information surface. Then, the focalpoint of the optical beam passes the information surface. The FE signalwhich is output from the FE signal generation circuit 2115 crosses zero.At this time, the zero-cross signal is output from zero-cross detectionmeans, i.e., the zero-cross detection circuit 2119.

[0136] The microcomputer 2122 judges that the focal point is on theinformation surface of the optical disc 2100 when the output from thecomparator 2128 is at the high-level and when the zero-cross signal isoutput from the zero-cross detection circuit 2119. In this case, themicrocomputer 2122 makes the potential at the control terminal d highand connects the terminal c and the terminal a of the switch 2117 tostart the focus control operation.

[0137] The microcomputer 2122 controls rotations of the optical disc2100 so that the number of rotations of the optical disc 2100 when theamplification detection circuit 2121 detects the amplification of the TEsignal is smaller than the number of the rotations of the optical disc2100 when the information recorded on the information surface of theoptical disc 2100 is being reproduced. Such a control is achieved by,for example, by controlling the number of rotations of rotation meansfor rotating the optical disc, i.e., the motor 2127. The microcomputer2122 increases the number of rotations of the motor 2127 to the normalnumber of rotations for reproducing information after the focus controlhas been started. By lowering the number of rotations of the opticaldisc 2100 when the amplification of the TE signal is being detected asdescribed above, the speed of in the focusing direction to theinformation surface, which may be increased due to the wobble of theoptical disc 2100, can be decreased. Accordingly, the time period duringwhich the information surface is in the depth of focus can be longer,and thus the number of tracks across which the optical beam 2106traverses can be increased. As a result, it is possible to detect theamplification of the TE signal accurately.

[0138] The microcomputer 2122 controls the strength of the optical beamsuch that the optical beam when the amplification detection circuit 2121detects the amplification of the TE signal is smaller than the strengthof the optical beam when the information recorded on the informationsurface of the optical disc 2100 is being reproduced. Such a control isachieved by controlling the light-emitting power of the laser 2109. Themicrocomputer 2122 increases the light-emitting power of the laser 2109to the normal power for reproducing the information after the focuscontrol has been started. By lowering the power of the optical beam whenthe amplification of the TE signal is being detected as described above,the information recorded on the optical disc 2100 can be prevented frombeing destroyed.

[0139]FIG. 5 shows an example of a change in the FE signal. In FIG. 5, ahorizontal axis indicates a distance between the focal point of theoptical beam 2106 focused with the objective lens 2103 and theinformation surface of the optical disc 2100. A vertical axis indicatesthe level of the FE signal. The FE signal has a waveform similar to ans-shape. Hereinafter, the waveform is referred to as the s-shapewaveform. The zero level of the FE signal indicates that the focal pointof the optical beam matches the information surface (i.e., is focused).The level of the FE signal is at the maximum value when the distance isabout 10 μm. As the distance becomes longer, the FE signal comes closerto zero. Thus, before the focus control operation, it is required toperform an initial operation for the focus control to bring the distancebetween the focal point of the optical beam 2106 and the informationsurface in the range L of FIG. 5.

[0140]FIG. 6 schematically shows the tracks formed on the optical disc2100. The optical beam 2106 is applied from the lower side in thefigure. The tracks are convex portions with respect to the lower side inthe figure. In FIG. 6, the information surface of the optical disc 2100is indicated by the reference numeral 2101 and the surface of theoptical disc 2100 is indicated by the reference numeral 2102.

[0141] The tracking error detection scheme which is generally called apush-pull method is described. The push-pull method is also referred toas a far-field method. In this method, the TE signal is detected by adifference in outputs from the light-receiving sections of thephotodetector divided in two and positioned symmetrically with respectto the center of the tracks, which receives an optical beam reflectedand diffracted with the tracks on the optical disc 2100.

[0142]FIG. 7 shows a waveform of the TE signal when the optical beam2106 traverses the tracks. When the optical beam 2106 traverses thetracks, the TE signal is in the sine waveform. The TE signal is zero atthe center of each of the tracks.

[0143]FIG. 8 shows waveforms of the signals when the objective lens 2103gradually approaches the information surface of the optical disc 2100.In FIG. 8, waveform (a) represents the output of the ramp generationcircuit 2123, waveform (b) represents the focal point, waveform (c)represents the FE signal, waveform (d) represents the zero-cross signal,waveform (e) represents the TE signal, waveform (f) represents an outputfrom the amplification detection circuit 2121, and waveform (g)represents the output of the comparator 2128.

[0144] When the microcomputer 2122 starts the operation of the rampgeneration circuit 2123 at time t₀, a current corresponding there toflows through the focusing coil. Thus, the objective lens 2103 graduallyapproaches the information surface of the optical disc 2100. Thezero-cross signal is output when the focal position matches the surfaceof the optical disc at time t₁. However, the level of the TE signal iszero at the optical disc surface. Thus, the output of the comparator2128 remains at the low level. Further, as the focal position furtherapproaches the optical disc 2100, the information surface enters thedepth of focus. Thus, the TE signal is in the sine waveform.Accordingly, the output from the amplification detection circuit 2121exceeds E₁ and the output of the comparator 2128 becomes high.

[0145] At time t₃, the zero-cross signal is output when the focal pointmatches the information surface. As the objective lens 2103 is furtherraised, the information surface goes out of the depth of focus. Thus,the TE signal reaches the zero level. At time t₄, the output from thecomparator 2128 becomes low. As described above, the zero-cross signalis output at the surface of the optical disc 2100. However, since theoutput of the comparator 2128 is low, the information surface can bedetected surely. Specifically, if the unit has the structure in whichthe microcomputer 2122 makes the potential at the control terminal dhigh to connect the terminal c and the terminal a of the switch 2117 attime t₃, the focusing to the information surface can be performed surelyeven if the reflectance of the information surface is low such as in thedoublelayer optical disc.

[0146]FIG. 9 shows waveforms of the signals when the focusing is beingperformed. In FIG. 9, waveform (a) represents the output from the rampgeneration circuit 2123, waveform (b) represents the focal point,waveform (c) represents the FE signal, waveform (d) represents thezero-cross signal, waveform (e) represents the TE signal, waveform (f)represents the output from the amplification detection circuit 2121 andthe waveform (g) represents the output from the comparator 2128.

[0147] Waveform (h) represents the control signal which is output to thecontrol terminal d of the switch 2117 by the microcomputer 2122. At t₁₀,the ramp generation circuit 2123 starts operation. At time t₁₂, theoutput from the comparator 2128 becomes high.

[0148] At time t₁₃, the focal point matches the information surface andthe zero-cross signal is output. The microcomputer 2122 makes thepotential at the control terminal d of the switch 2117 high.

[0149] Therefore, the terminal c and the terminal a of the switch 2117is connected and the focus control is operated. The focus control of theobjective lens 2103 is performed such that the focal point is on theinformation surface.

[0150] The FE signal also crosses zero at the surface of the opticaldisc 2100. However, the level of the TE signal at the surface of theoptical disc 2100 is zero. Accordingly, the microcomputer 2122 does notactivate the focus control. Thus, it is possible to perform the accuratefocusing to the information surface.

[0151] In the case where the optical disc 2100 is a disc on which theinformation is prerecorded (for example, a ROM), the information surfacemay be detected based on an RF signal. Such detection can be achieved byadding a total internal reflection detection circuit and an RF detectioncircuit to the structure of the optical disc unit 2002 shown in FIG. 4.

[0152] (Embodiment 3)

[0153]FIG. 10 shows an exemplary structure of an optical disc unit 2003of Embodiment 3. In FIG. 10, like blocks as in the above embodiments areindicated by like reference numerals, and the explanations thereof areomitted.

[0154]FIG. 11 shows a plurality of tracks formed on an informationsurface of an optical disc 2150. Each of the tracks is wavy. In theexemplary structure shown in FIG. 11, each of the tracks slightlywobbles in a radial direction thereof with a predetermined period W.These slight wobbles can be detected by the TE signals as a misalignmentbetween the optical beam 2106 and the tracks. The optical disc 2150 maybe a single-layer disc, or a multilayer disc, including a doublelayerdisc.

[0155]FIG. 12 shows waveforms of the signals when the optical beam 2106traverses the tracks with the focus control being performed, wherein (a)schematically represents the tracks. Waveform (b) represents a TEsignal. Waveform (c) represents an output from a BPF 2151. Hereinafter,the output from the BPF 2151 is referred to as a wobble signal.Amplification of the wobble signal is maximum when the optical beam 2016locates at the center of a track and small when the optical beam 2016 isbetween the tracks. The BPF 2151 passes components due to slight wobblesin the radial direction of the tracks included in the TE signal.Accordingly, a pass band of the BPF 2151 depends on W and the number ofrotations of the optical disc 2150.

[0156]FIG. 13 shows waveforms of the signals used in the optical discunit 2003. Waveform (a) represents the output from the ramp generationcircuit 2123, waveform (b) represents a focal point, waveform (c)represents the FE signal, waveform (d) represents a zero-cross signal,waveform (e) represents the wobble signal, waveform (f) represents theoutput from an amplification detection circuit 2121, the waveform (g)represents the output from a comparator 2460 and waveform (h) representsthe signal of the control terminal d of the switch 2117. Themicrocomputer 2122 activates the ramp generation circuit 2123 at timet₂₀, and a current corresponding thereto is supplied to the focusingcoil.

[0157] As described above, the objective lens 2103 gradually approachesthe information surface of the optical disc 2150. At time t₂₁, when thefocal point matches a surface of the optical disc 2150, the zero-crosssignal is output. Since the level of the wobble signal is zero, theoutput from the comparator 2460 remains low. When the focal pointfurther approaches the optical disc, at time t₂₂ the information surfaceenters the depth of focus. Thus, the wobble signals become the sine waveform signals. The output from the amplification detection circuit 2121exceeds E₂, and thus, the output from the comparator 2460 becomes high.At time t₂₃, when the focal point matches the information surface, thezero-cross signal is output. The microcomputer 2122 makes the potentialat the control terminal d of the switch 2117 high and connects theterminal c and the terminal a to perform the focus control.

[0158] At the surface of the optical disc 2150, the zero-cross signal isoutput. However, the output from the comparator 2460 is low. Thus, themicrocomputer 2122 keeps the level of the potential of the controlterminal low. In the switch 2117, the terminal b and the terminal careconnected and the focus control is not performed. On the other hand, atthe information surface of the optical disc 2150, the zero-cross signalis also detected. In this case, the output from the comparator 2460 ishigh. Thus, the microcomputer 2122 makes the potential at the controlterminal d high. In the switch 2117, the terminal a and the terminal care connected and the focus control is performed.

[0159] With such a structure, even if the reflectance of the informationsurface is low as in the doublelayer optical disc, it is possible tosurely detect the information surface and to surely perform thefocusing.

[0160] (Embodiment 4)

[0161]FIG. 14 shows an exemplary structure of the optical disc unit 2004according to Embodiment 4 of the present invention. Like blocks as inthe above embodiments are indicated by like reference numerals, and theexplanations thereof are omitted.

[0162] A ramp generation circuit 2157 generates a signal varying at aconstant speed when the potential at the terminal a becomes high. Thepolarity of the speed is positive when the potential at the terminal bis high and negative when the potential is low. The optical disc 2100rotates at the predetermined number of rotations. A motor controlcircuit 2156 controls the motor 2127 so as to rotate with thepredetermined number of rotations. A laser control circuit 2155 controlsthe laser 2109 so as to emit light at a predetermined power.

[0163] The focusing operation will be described. A microcomputer 2158makes the potential at the control terminal d of the switch 2117 low,and connects the terminal c and the terminal b. Next, the microcomputer2158 makes the potentials of the terminal a and the terminal b of theramp generation circuit 2157 high. As a result, the ramp generationcircuit 2157 generates a signal of a positive polarity varying at aconstant speed. A current which corresponds to the output from the rampgeneration circuit 2157 flows through the focusing coil by the poweramplifier 2128. As a result, the objective lens 2103 moves toward theoptical disc 2100 (in an upper direction in the figure). When the focalpoint of the optical beam 2106 matches the surface of the optical disc2100, the first zero-cross signal is output from the zero-crossdetection circuit 2119.

[0164] The microcomputer 2158 changes the potential of the terminal b ofthe ramp generation circuit 2157 from high to low after a predeterminedtime period M₀ has lapsed from the time when the first zero-cross signalwas detected. As a result, the ramp generation circuit 2157 generates asignal of a negative polarity varying at a constant speed after thepredetermined time period M₀ has lapsed from the time when the firstzero-cross signal was detected. Thus, the objective lens 2103 moves in adirection away from the optical disc 2100 (in a lower direction in thefigure) and thus the focal point of the optical beam shifts in adirection toward the information surface of the optical disc 2100 (in alower direction in the figure).

[0165] The predetermined time period M₀ is set to be longer than thetime for the focal point of the optical beam to reach the informationsurface. Specifically, the predetermined time period M₀ is a time periodduring which the objective lens 2103 can further move in the samedirection as the objective lens 2103 moves when the first zero-crosssignal is detected by a predetermined distance larger than the thicknessof the protective layer of the optical disc 2100. The thickness of theprotective layer of the optical disc 2100 is the distance between thesurface of the optical disc 2100 and the information surface.

[0166] The focal point of the optical beam starts to shift toward theinformation surface after it passes through the information surface.When the focal point of the optical beam passes through the informationsurface again, the zero-cross detection circuit 2119 outputs the secondzero-cross signal. When the microcomputer 2158 detects that the secondzero-cross signal is output (i.e., the focus error signal crosses zerofor the second time), it makes the potential at the control terminal dhigh and connects the terminal c and the terminal a of the switch 2117to start the focus control.

[0167]FIG. 15 shows waveforms of the signals used in the optical discunit 2004. In FIG. 15, waveform (a) represents the output of the rampgeneration circuit 2157, waveform (b) represents the focal point,waveform (c) represents the signal of the terminal b of the rampgeneration circuit 2157, waveform (d) represents the FE signal, waveform(e) represents the zero-cross signal, and waveform (f) represents thesignal of the terminal d of the switch 2117. The microcomputer 2158starts the operation of the ramp generation circuit 2157 at time t₃₀, acurrent corresponding thereto flows through the focusing coil.

[0168] Accordingly, the objective lens 2103 gradually approaches theinformation surface of the optical disc 2100. At time t₃₁, when thefocal point matches the surface of the optical disc 2100, the zero-crosssignal is output.

[0169] The microcomputer 2158 sets the potential at the terminal b ofthe ramp generation circuit 2157 low when the time period M₀ has lapsedafter the first zero-cross signal was detected.

[0170] At time t₃₂, the focal point and the information surface matches.Thus, the zero-cross signal is output. The output from the rampgeneration circuit decreased at a constant speed from time t₃₃. Thus,the focal point gradually approaches the information surface. At timet₃₄ the focal point matches the information surface, and the zero-crosssignal is output. The microcomputer 2158 makes the potential at thecontrol terminal d of the switch 2117 high and connects the terminal cand the terminal a to start the focus control operation.

[0171] With such a structure, even if the reflectance of the informationsurface is low as in the doublelayer optical disc, it is possible tosurely detect the information surface and to surely perform the focusingwithout requiring the TE signal.

[0172] In the present scheme, the focal point is once shifted above theinformation surface. Thus, it is not affected by the zero-cross signalat the surface of the optical disc 2100.

[0173] The distance by which the focal point approaches the optical disc2100 is limited relative to the surface of the optical disc 2100. Thus,the objective lens 2103 does not collide into the surface of the opticaldisc 2100. The predetermined time period M₀ depends on the sensitivityof the focus actuator and the rate of change of the output signal fromthe ramp generation circuit 2157.

[0174] In the present embodiment, the time period M₀ has thepredetermined amount. However, it may vary depending on a length of atime from time t₃₁ to time t₃₂. The distance between the surface of theoptical disc 2100 and the information surface is predetermined. Thus,the time for moving the distance is proportional to the sensitivity ofthe actuator.

[0175] Accordingly, even if the sensitivity of the actuator changes, itis possible to accurately shift the focal point to above the informationsurface.

[0176] (Embodiment 5)

[0177]FIG. 16 shows an exemplary structure of an optical disc unit 2005according to Embodiment 5 of the present invention. Like blocks as inthe above embodiments are indicated by like reference numerals, and theexplanations thereof are omitted.

[0178] The focusing operation will be described. A microcomputer 2160sets zero as the number of rotations of the motor to the motor controlcircuit 2126. The laser control circuit 2155 controls the laser 2109 toemit light at a predetermined power. The microcomputer 2160 changes thepotential at the control terminal d of the switch 2117 to low to connectthe terminal c and the terminal b. Then, the microcomputer 2160 changesthe potentials at both the terminal a and the terminal b of the rampgeneration circuit 2157 to high. As a result, the ramp generationcircuit 2157 generates a signal of a positive polarity varying at aconstant speed. A current corresponding to the output of the rampgeneration circuit 2157 flows through the focusing coil by a poweramplifier 2118. As a result, the objective lens 2103 shifts toward theoptical disc 2100 (in an upper direction in the figure).

[0179] The microcomputer 2160 changes the potential at the terminal b ofthe ramp generation circuit 2157 from high to low after a predeterminedtime period M₁ has lapsed after the first zero-cross signal wasdetected. Thus, the ramp generation circuit 2157 generates a signal of anegative polarity varying at a constant speed after the predeterminedtime period M₁ has lapsed since the first zero-cross signal wasdetected. As a result, the objective lens 2103 gradually recedes fromthe optical disc 2100.

[0180] The predetermined time period M₁ is set to be sufficiently longerthan the time for the focal point to reach the information surface.Specifically, the predetermined time period M₁ is set to be a timeperiod during which the objective lens 2103 can move by a distancelarger than the thickness of the protective layer of the optical disc2100. The microcomputer 2160 makes the potential at the control terminald of the switch 2117 high when the first zero-cross signal after thepotential at the terminal b of the ramp generation circuit 2157 ischanged to low is detected, and connects the terminal c and the terminala of the switch 2117 to start the focus control. The microcomputer 2160activates the sine wave generation circuit 2124. If the output from thecomparator 2128 is high, the microcomputer 2160 stops the operation ofthe sine wave generation circuit 2124 and sets a predetermined number ofrotations to the motor control circuit 2126. If the output from thecomparator 2128 is low, the ramp generation circuit 2157 is reset andthe terminal c and the terminal b of the switch 2117 are connected toperform the focusing again.

[0181]FIG. 17 shows waveforms of the signals used in the optical discunit 2005. Waveform (a) represents the output from a ramp generationcircuit 2157, waveform (b) represents a focal point, waveform (c)represents a signal at the terminal b of the ramp generation circuit2157, waveform (d) represents the FE signal, waveform (e) represents thezero-cross signal, waveform (f) represents the TE signal, waveform (g)represents a signal at the control terminal d of the switch 2117,waveform (h) represents the output from the comparator 2128, andwaveform (i) represents a signal corresponding to the predeterminednumber of rotations sent by the motor control circuit 2126 to the motor2127.

[0182] After the microcomputer 2160 starts the operation of the rampgeneration circuit 2157 at time t₄₀, a current corresponding theretoflows through the focusing coil. Thus, the objective lens 2103 graduallyapproaches the information surface of the optical disc 2100 and passesthrough the information surface. The microcomputer 2160 sets thepotential of the terminal b of the ramp generation circuit 2157 low attime t₄₁ when a predetermined time period M₁ has lapsed after time t₄₀.Then, the output from the ramp generation circuit 2157 starts todecrease at a constant speed from time t₄₁. Accordingly, the focal pointgradually approaches the information surface and matches the informationsurface at time t₄₂. The zero-cross signal is output.

[0183] The microcomputer 2160 makes the potential at the controlterminal d of the switch 2117 high and connects the terminal c and theterminal a of the switch 2117 to perform focus control. Themicrocomputer 2160 activates the sine wave generation circuit 2124 attime t₄₃. If the focal point of the optical beam is on the informationsurface, the optical beam traverses the tracks and the TE signal is inthe sine wave form. If amplification detection means, i.e., theamplification detection circuit 2121, detects that the amplification ofthe TE signal is a predetermined value or more, the output from thecomparator 2128 becomes high. The microcomputer 2160 determines that thefocusing to the information surface is normally ended and sets thepredetermined number of rotations to the motor control circuit 2126 attime t₄₄.

[0184] According to the present embodiment, whether the focusing isnormally ended is determined before the motor 2127 is rotated. Thus, themotor 2127 is not rotated when the focusing is not normally performedand thus in the case where the objective lens 2103 collided into thesurface of the optical disc 2100. Therefore, the optical disc 2100 isfree from being damaged in a wide range. Further, even if thereflectance of information surface is low as in the doublelayer opticaldisc, it is possible to surely detect the information surface and tosurely perform the focusing.

[0185] The rotation control of the motor 2127 described in the presentembodiment can be applied to any of the above-described embodiments.

[0186] (Embodiment 6)

[0187]FIG. 18 shows an exemplary structure of an optical disc unit 2006according to Embodiment 6 of the present invention. Like blocks as inthe above embodiments are indicated by like reference numerals, and theexplanations thereof are omitted.

[0188] In the present embodiment, a single rotation memory 2166 acts asstorage means for storing displacement in a focusing direction of theactuator 2104 which corresponds to a rotation angle of an optical disc2100.

[0189] The motor control circuit 2156 controls the motor 2127 to rotateat a predetermined number of rotations. The laser control circuit 2155controls the laser 2109 to emit light at a predetermined power. Arotation angle detection circuit 2165 detects and outputs a rotationangle of the motor 2127. Hereinafter, the signal is referred to as therotation angle signal. The single rotation memory 2166 stores an inputvoltage of the power amplifier 2118 during a cycle of a single rotationof the optical disc 2100 in synchronization with the rotation anglesignal. The stored value is output to an adder 2167 in synchronizationwith the rotation angle signal.

[0190] Such storage and output operations are controlled by amicrocomputer 2168.

[0191] In the structure of the present embodiment, an open loop gain ofa focus control system can be measured.

[0192] The microcomputer 2168 sends a sine wave to the adder 2167 withthe terminal c and the terminal a of the switch 2117 being closed andthe focus control is being performed. An objective lens 2103 iscontrolled so as to follow the sine wave added to the focus controlsystem. The microcomputer 2168 captures the FE signal in this state andcalculates the open loop gain of the focus control system based on therelationship between the added sine wave and amplification and a phaseof the FE signal. Based on the calculated gain value, the gain of theamplifier 2400 is changed so that the open loop has a predeterminedgain. The predetermined gain is a gain assumed when a phase leadcharacteristic of a phase compensation circuit 2170 which will bedescribed later is designed.

[0193] The phase compensation circuit 2170 is a filter for causing thephase to lead for stabilizing the focus control system.

[0194] In this structure, a phase characteristic can be switched to leadin a wide band or a narrow band. The focusing is performed with thephase characteristic set to lead in the wide band. Then, after the openloop gain of the focus control system is adjusted, the phasecharacteristic of the band is set to lead in the narrow band.Specifically, a period between the time when the focusing starts and thetime when the open loop gain of the focus control system is adjusted,the phase characteristic is set to lead in the wide band. Due tovariance in reflectances of the optical disc 2100 and variance insensitivities of the focus actuator, the open loop gain deviates fromthe predetermined gain. Thus, when the focusing is performed, the focuscontrol is performed with the phase characteristic set to lead in thewide band, and after a gain adjustment, it is returned to the statewhere the phase leads in a normal band.

[0195] Therefore, the focusing becomes stable, and it is possible to setthe open loop gain after the gain adjustment higher than the open loopgain at the focusing.

[0196] The phase compensation circuit 2170 is described with referenceto FIGS. 19 and 20.

[0197]FIG. 19 shows an exemplary structure of the phase compensationcircuit 2170. A first input terminal 2300 is connected to a first phasecompensation circuit 2301 and a third phase compensation circuit 2303.The first phase compensation circuit 2301 and a second phasecompensation circuit 2302 are connected in series. An output of thesecond phase compensation circuit 2302 is connected to a terminal a of aswitch 2304. The third phase compensation circuit 2303 is connected inparallel with the first phase compensation circuit 2301 and the secondphase compensation circuit 2302 connected in series. The output thereofis connected to a terminal b of the switch 2304. A terminal c of theswitch 2304 is connected to an output terminal 2306. A signal from theoutput terminal 2306 is input to the single-rotation memory 2166 and theadder 2167. A terminal d of the switch 2304 is connected to a secondinput terminal 2305. The second input terminal 2305 is connected to themicrocomputer 2168.

[0198]FIG. 20 shows phase characteristics of the circuits included inthe phase compensation circuit 2170. In FIG. 20, the horizontal axisindicates frequencies and vertical axis indicates phases. Thefrequencies indicated by the horizontal axis are scaled by logarithm.

[0199] (a) in FIG. 20 represents a phase characteristic of the firstphase compensation circuit 2301. In the first phase compensation circuit2301, the phase leads within the band between the frequencies f₀ and f₃.

[0200] (b) in FIG. 20 represents a phase characteristic of the secondphase compensation circuit 2302. In the second phase compensationcircuit 2302, the phase leads within the band between the frequencies f₂and f₅.

[0201] (c) in FIG. 20 represents a phase characteristic of the firstphase compensation circuit 2301 and the second phase compensationcircuit 2302 connected in series. In this serial circuit, the phaseleads within the band between the frequencies f₀ and f₅.

[0202] (d) in FIG. 20 represents a phase characteristic of the thirdphase compensation circuit 2303. In the third phase compensation circuit2303, the phase leads within the band between the frequencies f₁ and f₄.

[0203] Thus, by switching the level of the second input terminal, thephase characteristic can be switched to lead in the wide band or in thenarrow band.

[0204] The frequency at which the gain of the open loop becomes 0 dB ispredetermined to be between f₂ and f₃. Thus, the phase characteristic ofthe first phase compensation circuit 2301 and the second phasecompensation circuit 2302 connected in series is designed such that thephase leading becomes the maximum between frequencies f₂ and f₃. Thephase characteristic of the third phase compensation circuit 2303 isalso designed such that the phase leading becomes the maximum betweenfrequencies f₂ and f₃. The phase compensation circuit of the seriallyconnected first phase compensation circuit 2301 and the second phasecompensation circuit 2302 has a wider band for the phase to leadcompared to the third phase compensation circuit 2303. Thus, even if theopen loop gain varies, a phase allowance can be secured, and thus thecontrol system is stable. However, widening the band for phase to leadresults in an increase in the gain of the phase compensation circuit2170. Thus, an excessive amount of current flows through the coil of theactuator due to noise or the like. In order to prevent this excessivecurrent, when the serially connected first phase compensation circuit2301 and the second phase compensation circuit 2302 are used, it ispreferable to reduce the open loop gain a little.

[0205]FIG. 21 shows waveforms when the focusing is performed. In FIG.21, waveform (a) represents the output from a ramp generation circuit2123, waveform (b) represents a focal point, waveform (c) represents aninput waveform of the comparator 2128, waveform (d) represents an outputwaveform of the single-rotation memory 2166, waveform (e) represents theFE signal, waveform (f) represents a zero-cross signal, and waveform (g)represents a signal at the control terminal d of the switch 2117. Themicrocomputer 2168 makes the potential at the second input terminal 2305of the switch 2304 high and connects the terminal a and the terminal csuch that the output signal from the serially connected first phasecompensation circuit 2301 and the second phase compensation circuit 2302is transmitted to the output terminal 2306. Thus, the phasecharacteristic of the phase compensation circuit 2170 becomes the onehaving the wide band for the phase to lead.

[0206] The microcomputer 2168 makes the potential of the controlterminal d of the switch 2117 low at time t₅₀ and connects the terminalc and the terminal b of the switch 2117. The ramp generation circuit2123 generates a signal varying at a constant speed. A currentcorresponding to the output of the ramp generation circuit 2123 flowsthrough the focusing coil by the power amplifier 2118. Accordingly, theobjective lens 2103 moves toward the optical disc 2100 (in an upperdirection in the figure). When the focal point matches the surface ofthe optical disc 2100, the first zero-cross signal is output. Themicrocomputer 2168 makes the potential at the control terminal d of theswitch 2117 high at time t₅₁ when the first zero-cross signal isdetected, and connects the terminal a and the terminal c of the switch2117 to perform the focus control.

[0207] The focus control is performed such that the focal point of theoptical beam locates on the surface of the optical disc 2100.

[0208] Since the optical disc 2100 wobbles, the objective lens 2103moves up and down to follow the wobbles. Accordingly, the input level ofthe power amplification circuit 2118 before the single-rotation memory2166 is activated is proportional to the wobbles.

[0209] The time period from time t₅₁ to t₅₂ is a cycle of a singlerotation of the optical disc 2100. The microcomputer 2168 commands thesingle-rotation memory 2166 to operate storage at time t₅₁. Thesingle-rotation memory 2166 stores the level at the terminal b of theswitch 2117 from t₅₁ to t₅₂. Then, the single-rotation memory 2166outputs the stored value to the adder 2167 at time t₅₂ and after. Themicrocomputer 2168 makes the potential at the control terminal d of theswitch 2117 low at time t₅₂, thereby connecting the terminal c and theterminal b of the switch 2117. The microcomputer 2168 makes thepotentials at the terminal a and the terminal b of the ramp generationcircuit 2123 high, and sends a command for reactivating the rampgeneration circuit 2123. Therefore, the output from the adder 2167 is asignal which is obtained by adding the output of the ramp generationcircuit 2123 and the output of the single-rotation memory 2166. Theobjective lens 2103 gradually approaches the optical disc 2100 inresponse to the output from the adder 2167.

[0210] When the focal point matches the information surface at time t₅₃,the zero-cross signal is output. The microcomputer 2168 stops the outputfrom the single-rotation memory 2166, makes the potential at the controlterminal d of the switch 2117 high, and connects the terminal c and theterminal a of the switch 2117. Thus, the focus control is performed suchthat the focal point is on the information surface.

[0211] The microcomputer 2168 performs a gain adjustment, and the gainof the amplifier 2400 is changed such that the open loop gain of thefocus control becomes the predetermined value.

[0212] The microcomputer 2168 makes the potential at the second inputterminal 2305 of the switch 2304 low and connects the terminal b and theterminal c of the switch 2304 such that the output signal of the thirdphase compensation circuit 2303 is output. Thus, the phasecharacteristic of the phase compensation circuit 2170 is switched tolead in the narrow band.

[0213] According to the present embodiment, even if the optical disc2100 has wobbles larger than the working distance, the objective lens2103 and the optical disc 2100 do not collide.

[0214]FIG. 22 illustrates the working distance. The working distance isthe shortest distance K between the surface of the optical disc 2100 andthe upper surface of the objective lens 2103 when the focal point is onthe information surface.

[0215] According to the present embodiment, the relative speed of theobjective lens 2103 and the information surface of the optical disc 2100is reduced to substantially zero. Thus, the focusing is stabilized.

[0216] As factors of variance in the open loop gain, variances inreflectance of the information surface of the optical disc 2100 and inthe sensitivity of the focus actuator are described. In the case of adoublelayer optical disc, the amplification of the FE signal normalizedby reflectance amount changes due to the light reflected off anotherinformation surface and thus the open loop gain varies. According to thepresent embodiment, the open loop gain is adjusted. Thus, even if thereflectance of information surface is low as in the doublelayer opticaldisc, it is possible to surely detect the information surface and tosurely perform the focusing.

[0217] (Embodiment 7)

[0218]FIG. 23 shows an exemplary structure of an optical disc unit 2007according to Embodiment 7 of the present invention. Like blocks as inthe above embodiments are indicated by like reference numerals, and theexplanations thereof are omitted.

[0219] In the present embodiment, a photodetector 2113 acts as aphotodetection means for detecting the light reflected off an opticaldisc 2187 having a plurality of information surfaces, after the opticalbeam is focused and applied to a predetermined information surface ofthe optical disc 2187.

[0220] The FE signal generation circuit 2115 acts as focus errordetection means for detecting a misalignment between the focal point ofthe optical beam and the predetermined information surface of theoptical disc 2187 based on an output from the photodetector 2113.

[0221] A total internal reflection amount signal generation circuit 2183acts as total internal reflection amount detection means for detecting atotal internal reflection amount from the optical disc 2187 based on theoutput from the photodetector 2113.

[0222] A divider 2185 acts as normalization means for dividing theoutput of the focus error detection means by a value obtained bysubtracting a signal value corresponding to the reflection amount of thelight reflected off the information surfaces other than thepredetermined information surface of the optical disc from the output ofthe total internal reflection amount detection means.

[0223] The optical disc 2187 is a doublelayer optical disc having twoinformation surfaces, i.e., a first information surface and a secondinformation surface, on one side. The motor control circuit 2156controls the motor 2127 to rotate at a predetermined number ofrotations. The laser control circuit 2155 controls the laser 2109 so asto emit light at a predetermined power. The light reflected off theoptical disc 2187 incident on the photodetector 2113 and is sent to thefocus error signal generation circuit 2115, and the total internalreflectance amount signal generation circuit 2183. The total internalreflectance amount signal generation circuit 2183 detects and outputsthe total internal reflectance reflected off the optical disc 2187 andincidents on the photodetector 2113. Hereinafter, the output from totalinternal reflectance amount signal generation circuit 2183 is referredto as total internal reflectance amount signal.

[0224] The total internal reflectance amount signal is sent to theterminal b of the divider 2185 via a subtractor 2184. An FE signal isinput to the terminal a of the divider 2185. The divider 2185 dividesthe signal input to the terminal a by the signal input to the terminal band then outputs from the terminal c. The divider 2185 maintains theconstant level of the FE signal without the amplification level of theFE signal being affected by the changes in the reflectances of theinformation surfaces of the optical disc 2187. Hereinafter, the outputof the divider 2185 is referred to as the normalization FE signal. Theoutput of the divider 2185 is sent to the power amplifier 2118 via thephase compensation circuit 2116 and the switch 2117.

[0225] Thus, even when the reflectance of the information surface of theoptical disc 2187 is changed, the gain of the open loop does not change.However, in the doublelayer optical disc, light reflected off theinformation surfaces other than the information surface on which thefocal point locates incidents on the photodetector 2113. Thus, eventhough the FE signal is normalized with the total internal reflectance,the level of the FE signal lowers. The subtractor 2184 compensates forthe amount of the light reflected off other information surfaces. Aswitch 2186 is connected to the subtractor 2184. A first referencevoltage 2181 and a second reference voltage 2182 are connected to theswitch 2186. The switch 2186 outputs a signal of either of them by acommand from the microcomputer 2180.

[0226] The first reference voltage 2181 corresponds to an amount oflight reflected off the second information surface in the case where thefocal point is on the first information surface. The second referencevoltage 2182 corresponds to an amount of light reflected off the firstinformation surface in the case where the focal point is on the secondinformation surface. Accordingly, the subtractor 2185 outputs a signalrepresenting the reflectance amount of the first information surface orsecond information surface from which the amount of light reflected offthe other information surface is removed.

[0227] The level of the first reference voltage 2181 and the secondreference voltage 2182 depends on the characteristic of the optical head2114, reflectance of the optical disc 2187, and the like. When the focalpoint of the optical beam is shifted from the information surface to thesecond information surface, the microcomputer 2180 makes the controlterminal d of the switch 2117 low and connects the terminal c and theterminal b of the switch 2117.

[0228] The microcomputer 2180 sends a drive voltage of the focusing coilfor moving the focal point of the optical beam from the firstinformation surface to the second information surface via a D/Aconverter. After the focal point is shifted, the control terminal d ofthe switch 2117 is changed to be high again. The terminal c and theterminal a are connected to operate the focus control. As describedabove, the switch 2186 is switched depending on whether the focal pointof the optical beam is on the first information surface or on the secondinformation surface. Thus, it is possible to change a signal valuecorresponding to the amount of light reflected off the informationsurfaces other than the information surface of the optical disc 2187 onwhich the focal point of the optical beam locates, which is input to thenormalization means, i.e., the subtractor 2185, in accordance with theinformation surface (i.e., the signal value which is input to theterminal b of the subtractor 2185).

[0229]FIG. 24 illustrates the doublelayer optical disc 2187 and theoptical beam 2106. In the example shown in Figure 24, the focal point ison the first information surface. In the case of reproducing theinformation recorded on the first information surface, the focus controlis performed such that the focal point is on the first informationsurface. In the case of reproducing information recorded on the secondinformation surface, the focus control is stopped, the objective lens2103 is brought closer to the optical disc 2187, and after the focalpoint is shifted to the second information surface, the focus control isperformed again.

[0230] In the case where the focal point is on the first informationsurface, the optical beam L1 is reflected off the first informationsurface and incidents on the photodetector 2113. The FE signal isgenerated by the optical beam L1.

[0231] However, a portion of the optical beam L2 transmitted through thefirst information surface and reflected of f the second informationsurface incidents on the photodetector 2113. The reflected light doesnot affect the FE signal but increases the total internal reflectionamount signal. Thus, when the FE signal is normalized with the totalinternal reflection amount signal, the level of the FE signal isdecreased by the amount of the optical beam L2. The amounts of the lightreflected off the other information surface are different in the casewhere the focal point is on the first information surface and in thecase where the focal point is on the second information surface.

[0232] Next, the operation for moving the focal point from the firstinformation surface to the second information surface is described.

[0233]FIG. 25 shows waveforms of the signals used in the optical discunit 2007. Waveform (a) represents the FE signal after normalization,waveform (b) represents an output waveform of D/A converter of themicrocomputer 2180, waveform (c) represents a waveform from the terminald of the switch 2186, and waveform (d) represents a signal output to theterminal d of the switch 2117. The microcomputer 2180 outputs anacceleration pulse for moving the focal point to the second informationsurface from time t₆₀ via the D/A converter. Thus, the objective lens2103 moves toward the second information surface and the focal pointalso shifts toward the second information surface. The microcomputer2180 detects that the level of the normalized FE signal becomes −E₃ attime t₆₁ and stops the acceleration pulse. When the normalized FE signalcrosses zero at time t₆₂, the terminal c of the switch 2186 is switchedfrom the terminal a to the terminal b and connected thereto. When thelevel of the normalized FE signal is E₃ at time t₆₃ the decelerate pulseis output. The decelerate pulse is output during the period in which thelevel of the normalized FE signal is E₃ or higher, i.e., until time t₆₄.

[0234] The microcomputer 2180 connects the terminal c and the terminal aof the switch 2117 when the normalized FE signal crosses zero at timet₆₅ and the focus control is performed again. The time when theaccelerate pulse and the decelerate pulse are output is controlled basedon the FE signal normalized with the total internal reflection amountsignal from which the amount of light reflected off the otherinformation surfaces is removed. Therefore, it is possible to detect thetiming accurately and the focal point can shift between the informationsurfaces stably.

[0235] (Embodiment 8)

[0236]FIG. 26 shows an exemplary structure of an optical disc unit 2008according to Embodiment 8 of the present invention. Like blocks as inthe above embodiments are indicated by like reference numerals, and theexplanations thereof are omitted.

[0237] A photodetector 2188 has five light-receiving sections. Withrespect to Embodiment 2, the photodetector 2113 provided with 4light-receiving sections is described. In the present embodiment, alight-receiving section surrounding the outside of the 4 light-receivingsections is further provided and acts as stray light detection means fordetecting light reflected off information surfaces other than thepredetermined information surface of the optical disc.

[0238] In the present embodiment, the photodetector 2188 is composed ofthe 4 light-receiving sections which form the photodetector 2113described with reference to Embodiment 2, and the stray light detectionmeans, i.e., the light receiving section provided so as to surround theoutside of the 4 light-receiving sections. The total light amount of thelight receiving sections located inside the light-receiving sectionwhich is the stray light detection means is a total internal reflectionamount signal. This is the amount of the light received in the partcorresponding to the photodetector 2113 described in Embodiment 2.

[0239] As described with reference to FIG. 24, light reflected off thesecond surface in the case where the focal point is on the firstinformation surface incidents on the entire photodetector 2188. Most ofthe light reflected off the first information surface incidents on theinner light-receiving sections. Accordingly, the light reflected off thesecond information surface incident on the inner light-receiving sectionis proportional to the light amount incident on the outerlight-receiving section.

[0240] The value obtained by multiplying the light amount of the outerlight-receiving section by a predetermined coefficient K is subtractedfrom the total internal reflection amount signal by the subtractor 2184.Thus, the total internal reflection amount signal without an effect oflight reflected off other information surfaces is obtained.

[0241]FIG. 27 schematically shows the structure of the photodetector2188. The inner 4 light receiving sections correspond to thephotodetector 2113. The outer light-receiving section is the addedportion. The operation of shifting the focal point from the firstsurface to the second surface is similar to that of Embodiment 7, andthus the explanation is omitted.

[0242] (Embodiment 9)

[0243]FIG. 28 shows an exemplary structure of an optical disc unit 2009according to Embodiment 9 of the present invention. Like blocks as inthe above embodiments are indicated by like reference numerals, and theexplanations thereof are omitted.

[0244] The optical disc 2187 is a doublelayer optical disc having twoinformation surfaces on one side. The motor control circuit 2156controls the motor 2127 so as to rotate with a predetermined number ofrotations. The laser control circuit 2155 controls the laser 2109 so asto emit light at a predetermined power.

[0245] The operation for detecting light reflected off other informationsurfaces is described. A microcomputer 2195 changes level a the controlterminal e of a switch 2196 to connect a terminal d and a terminal b.Also, the microcomputer 2195 changes the level at a control terminal eof a switch 2410 to connect a terminal d and a terminal c. The terminalc of the switch 2410 is set to zero level. The microcomputer 2195activates the ramp generation circuit 2123. The output from the rampgeneration circuit 2123 is sent to the power amplifier 2118 via theswitch 2196. Thus, the objective lens 2103 approaches the optical disc2187. An s-shape waveform is first output at the surface of the opticaldisc 2187. Next, an s-shape waveform is output at the first informationsurface. Then, an s-shape waveform is output at the second informationsurface.

[0246] The microcomputer 2195 measures amplification H₁ of the s-shapewaveform at the first information surface and amplification H₂ of thes-shape waveform at the second information surface. The microcomputer2195 prestores amplification H_(S) of an s-shape waveform at asingle-layer optical disc and the level of a total internal reflectionamount signal C_(S) in the case where the focal point is on theinformation surface. The microcomputer 2195 sets Q₁ obtained fromformula (2) to the terminal a of the switch 2410 as light reflected offother information surfaces, at the first information surface. Q₂obtained from formula (3) is set to the terminal b of the switch 2410 aslight reflected off other information surfaces, at the secondinformation surface.

Q ₁ =C _(S)·(1−(H ₁ /H _(S)))  (2)

Q ₂ =C _(S)(1−(H ₂ /H _(S)))  (3)

[0247] After Q₁ and Q₂ are obtained, the focusing is performed again.Specifically, the microcomputer 2195 changes the level at the controlterminal e of the switch 2196 to connect terminal d and terminal b.Also, the microcomputer 2195 changes the level at the control terminal eof the switch 2410 to connect the terminal d and the terminal a. Themicrocomputer 2195 activates the ramp generation circuit 2123. Theoutput from the ramp generation circuit 2123 is sent to the poweramplifier 2118 via the switch 2196. Thus, the objective lens 2103approaches the optical disc 2187. When the microcomputer 2195 detectsthe first information surface, it changes the level of the controlterminal e of the switch 2196 and connects the terminal d and terminal ato start the focus control operation. In the case where the focal pointis shifted to the second information surface, the microcomputer 2195changes the level at the control terminal e of the switch 2196 toconnect the terminal d and the terminal c, and outputs an accelerationpulse to the terminal c of the switch 2196 via the D/A converter. Also,the microcomputer 2195 changes the level at the control terminal e ofthe switch 2410 to connect the terminal d and the terminal b. Themicrocomputer 2195 changes the level at the control terminal e of theswitch 2196 to connect the terminal d and terminal a again to start thefocus control operation. In other words, in the case where the focalpoint of the optical beam is shifted, the switch 2410 is switched inaccordance to the information surfaces. When the focal point is on thefirst information surface, the terminal a and the terminal d areconnected, and when the focal point is on the second informationsurface, the terminal b and the terminal d are connected.

[0248]FIG. 29 shows waveforms of the signals used in the optical discunit 2009. In FIG. 29, waveform (a) represents an output of the rampgeneration circuit 2123, waveform (b) represents the focal point, andwaveform (c) represents the normalized FE signal which is the output ofsubtractor 2185. The microcomputer 2195 activates the ramp generationcircuit 2123 at time t₇₀. Thus, the focal point approaches the opticaldisc 2187, and the level of the normalized FE signal at the surfaceexceeds E₄ at time t₇₀. Further, the focal point of the optical beamapproaches the optical disc 2187, and the level of the normalized FEsignal becomes lower than −E₄ at time t₇₂.

[0249] The microcomputer 2195 detects that the focal point of theoptical beam passes through the surface of the optical disc 2187. As theobjective lens 2103 is further raised, the level of the normalized FEsignal at the first information surface exceeds E₄ at time t₇₃. Themicrocomputer 2195 measures and stores the maximum value a₁ of thenormalized FE signal during the period until the level of the normalizedFE signal becomes E₄ again. At time t₇₄, the level of the normalized FEsignal becomes lower than −E₄. The microcomputer 2195 measures andstores the level of the minimum value b₁ of the normalized FE signalduring the period in which the level of the normalized FE signal becomes−E₄ again. b₁ is a negative value. The value obtained by subtracting b,from a₁ is amplification H₁ of the s-shape waveform at the firstinformation surface. As the objective lens 2103 is further raised, thefocal point of the optical beam further approaches the optical disc2187. At time t₇₅, the level of the normalized FE signal exceeds E₄. Themicrocomputer 2195 measures and stores the maximum value a₂ of thenormalized FE signal during the period until the level of the normalizedFE signal becomes E₄ again. At time t₇₆, the level of the normalized FEsignal becomes lower than −E₄. The microcomputer 2195 measures andstores the level of the minimum value b₂ of the normalized FE signalduring the period until the level of the normalized FE signal becomes−E₄ again. The value obtained by subtracting b₂ from a₂ is amplificationH₂ of the s-shape waveform at the second information surface.

[0250] The microcomputer 2195 calculates Q₁ and Q₂ using the above twoformulas. In Embodiment 9, the light amount reflected off the otherinformation surfaces is detected with the amplification of thenormalized FE signal when the focal point passes through the informationsurface. However, when the amplification of the normalized FE signaldecreases, the open loop gain of the focus control system decreasesproportionally. The focus gain measurement means (not shown) may be usedto measure the open loop gain of the focus control system, and based ona ratio of the measured gain and the gain for the single-layer opticaldisc, values of the terminal a and the terminal b of the switch 2410 maybe set.

[0251] (Embodiment 10)

[0252]FIG. 30 shows an exemplary structure of an optical disc unit 2010according to Embodiment 10 of the present invention. Like blocks as inthe above embodiments are indicated by like reference numerals, and theexplanations thereof are omitted.

[0253] In the present embodiment, a hologram element 2250 acts asoptical beam splitting means for splitting light which is reflected offthe optical disc 2187, after the optical beam is focused and applied toa predetermined information surface of the optical disc 2187, into lightof an inner region close to an optical axis and light of an outer regionfar from the optical axis.

[0254] An inner FE signal generation circuit 2258 acts as inner focuserror detection means for detecting a misalignment between the focalpoint of the optical beam and the predetermined information surface ofthe optical disc 2187 based on the light of the inner region. An outerFE signal generation circuit 2254 acts as outer focus error detectionmeans for detecting a misalignment between the focal point of theoptical beam and the predetermined information surface of the opticaldisc based on the light of the outer region.

[0255] The optical disc 2187 is a doublelayer disc having twoinformation surfaces on one side. The optical disc 2187 rotates at apredetermined number of rotations. The laser 2109 emits light at apredetermined power.

[0256] The light emitted from the laser 2109 becomes parallel light by acollimate lens 2430 and transmits through a beam splitter 2256.

[0257] The transmitted optical beam 2106 is condensed on the opticaldisc 2187 by the objective lens 2103 as condensing means. The condensedoptical beam is reflected/diffracted by the tracks on the optical disc2187.

[0258] The reflected/diffracted optical beam transmits the objectivelens 2103 again and is reflected off the beam splitter 2256.

[0259] The reflected optical beam 2106 is separated into diffractionlight and 0th order light by the hologram element 2250 as the opticalbeam splitting means. The 0th order light passes through the hologramelement 2250 is condensed by the detection lens 2111, is givenastigmatism of 450 relative to the tracks by the cylindrical lens 2112,and enters a photodetector 2253.

[0260] The photodetector 2253 receives the light and outputs a signal.The signal is input to a controlling FE signal generation circuit 2257.The controlling FE signal generation circuit 2257 generates acontrolling FE signal.

[0261] The controlling FE signal is sent to the power amplifier 2118 viathe phase compensation circuit 2116 and the switch 2117. Thus, a currentflows through the focusing coil in accordance with the controlling FEsignal.

[0262] +1st order light and−1st order light diffracted with the hologramelement 2250 is condensed by the detection lens 2111, is givenastigmatism of 450 relative to the tracks by the cylindrical lens 2112,and enters the photodetector 2253.

[0263] The photodetector 2253 receives the light, the optical beamreflected off the optical disc, divides it into the optical beam lightof the inner region closer to the optical axis and the optical beamlight of the outer region far from the optical axis, and outputssignals. The signals are respectively sent to the inner and outer FEsignal generation circuits 2258 and 2254.

[0264] In the doublelayer optical disc, each of the first and secondinformation layers has a protective layer of different thickness. Thus,spherical aberration is generated. The optical head is designed suchthat the spherical aberration is zero when the thickness of theprotective layer is that between the thicknesses of the protectivelayers of the first and the second information surfaces. Thus, in thefirst information surface, the thickness of the protective layer isthin, and in the second information surface, the thickness of theprotective layer is thick. Accordingly, the spherical aberrations at thefirst and the second information surfaces have reversed polarities.

[0265] Due to the spherical aberration, in the case where the focalpoint is on the first information surface (i.e., in the case where thelevel of the controlling FE signal is 0 at the first informationsurface), the level of the inner FE signal becomes positive and thelevel of the outer FE signal becomes negative.

[0266] In the case where the focal point is on the second informationsurface (i.e., in the case where the level of the controlling FE signalis 0 at the second information surface), the level of the inner FEsignal becomes negative and the level of the outer FE signal becomespositive.

[0267] When the focal point of the optical beam is shifted from thefirst information surface to the second information surface, theterminal c and the terminal b of the switch 2117 are connected.

[0268] The microcomputer 2255 sends a drive voltage of the focusing coilfor shifting the focal point of the optical beam from the firstinformation surface to the second information surface to the terminal bof the switch 2117 via the D/A converter. The focal point of the opticalbeam starts to shift toward the second information surface. Themicrocomputer 2255 stops an acceleration pulse when the outer FE signalcrosses zero and outputs a deceleration pulse.

[0269] When the focal point of the optical beam is shifted from thefirst information surface to the second information surface, the outerFE signal first crosses zero near the second information surface, andthen the controlling FE signal crosses zero. Then, when the outer FEsignal crosses zero again, the microcomputer 2255 stops the decelerationpulse.

[0270] Then, when the controlling FE signal crosses zero, the terminal cand the terminal a of the switch 2117 are connected. The focus controlis performed again.

[0271] Next, with reference to FIG. 31, the relationship between thespherical aberration and the focal point is described. FIG. 31illustrates the outer and inner focal points of the optical beam whenthe controlling FE signal is zero at the first information surface.

[0272] As described above, in the first information surface, thethickness of the protective layer is smaller than the optimal value. Thespherical aberration is as illustrated. The outer optical beam focuseson a position close to the objective lens 2103. The inner optical beamfocuses on a position far from the objective lens 2103.

[0273] In the case where the controlling signal is zero at the secondinformation surface, the thickness of the protective layer is largerthan the optimal value. Thus, the outer optical beam focuses on aposition far from the objective lens 2103. The inner optical beamfocuses on a position close to the objective lens 2103.

[0274] Thus, when the objective lens 2103 approaches the informationsurfaces, the outer FE signal and the inner FE signal are in thewaveforms as shown in FIG. 32. The solid line represents the inner FEsignal and the broken line represents the outer FE signal. Thecontrolling FE signal is an average of the outer FE signal and the innerFE signal.

[0275] As described above, when the focal point is shifted from thefirst information surface to the second information surface, the outerFE signal first crosses zero near the second information surface, andthen the controlling FE signal crosses zero.

[0276] Next, an operation for shifting the focal point from the firstinformation surface to the second information surface is described.

[0277]FIG. 33 shows waveforms of the signals used in the optical discunit 2010. Waveform (a) represents the FE signal, waveform (b)represents waveform at the terminal d of the control 2117, and waveform(c) represents an output of the D/A converter of the microcomputer 2255.In waveform (a), the broken line represents the outer FE signal, a thicksolid line represents the inner FE signal, and the fine solid linerepresents the controlling FE signal.

[0278] The microcomputer 2255 outputs the acceleration pulse forshifting the focal point to the second information surface from timet₇₀. Thus, the focal point shifts toward the second information surface.The microcomputer 2255 detects that the level of the outer FE signal iszero at time t₇₁ and stops the acceleration pulse. Then, themicrocomputer 2255 outputs the deceleration pulse.

[0279] The microcomputer 2255 stops the deceleration pulse at time t₇₂,and connects the terminal c and the terminal a of the switch 2117 attime t₇₁ when the controlling FE signal crosses zero to perform thefocus control again.

[0280] According to the optical disc unit of the present embodiment, itis possible to stop the deceleration pulse in an accurately timed mannercompared to the case where the deceleration pulse is stopped based onthe level of the controlling FE signal because the deceleration pulsecan be stopped at the time when the outer FE signal crosses zero. As aresult, it is possible to shift the focal point of the optical beam fromone information surface to another information surface stably.

[0281] In the case where the optical system is not designed such thatthe spherical aberration is zero when the thickness of the protectivelayer is that between the thicknesses of the protective layers of thefirst and second information surfaces, timing may be determined by usingthe inner FE signal.

[0282] In such a case, in accordance with the information on thespherical aberration of depending on the thickness of the protectivelayer which is between the thicknesses of the protective layers of thefirst and second information surfaces, at least one of the outer FEsignal and the inner FE signal maybe appropriately selected. Based onthis signal, the actuator 2104 may be driven and the focal point of theoptical beam may be shifted from one information surface to anotherinformation surface.

INDUSTRIAL APPLICABILITY

[0283] In an optical disc unit according the present invention, focuscontrol to an information recording layer of an optical disc isperformed after focus control to a surface of a protective layer of theoptical disc is performed. Thus, the working distance is substantiallyextended by the thickness of the protective layer. As a result, it ispossible to significantly reduce the possibility of an objective lenscolliding into the optical disc surface even when an optical head havinga large NA is used.

[0284] In another optical disc unit according to the present invention,only when amplification of a tracking error signal is detected to be apredetermined value or higher, the focus control is allowed to bestarted. Thus, without referring to the level of total internalreflection amount, it is possible to distinguish the optical discsurface and the information surface. As a result, even if the differencein levels of the total internal reflection amount of the optical discsurface and the total internal reflection amount of the informationsurface is small (for example, in the case of the doublelayer opticaldisc), the focusing to the information surface can be surely performed.

[0285] In another optical disc unit according to the present invention,in response to the detection of the focus error signal crossing zero forthe second time, the focus control to the information surface isstarted. Thus, without referring to the level of the total internalreflection amount, it is possible to distinguish the optical discsurface and the information surface. As a result, even if a differencein the levels of the total internal reflection amount of the opticaldisc surface and the total internal reflection amount of the informationsurface is small (for example, in the case of the doublelayer opticaldisc), the focusing to the information surface can be surely performed.

[0286] In another optical disc unit according to the present invention,after wobbles of the optical disc surface have been learnt, focuscontrol to the information surface is started. Thus, the focusingcontrol to the information surface is performed to the optical discsurface of which wobbles has been learnt. As a result, it is possible tosignificantly reduce the possibility of the objective lens collidinginto the optical disc due to wobbles of the optical disc.

[0287] In another optical disc unit according to the present invention,means for accurately calculating the total internal reflectance from theparticular information surface (normalization means) is provided. Thus,the effects of the light reflected off information surfaces other thanthe particular information surface can be removed.

1. An optical disc unit for an optical disc having one or moreinformation recording layers and one or more protective layers formed onthe information recording layers, comprising: reflective surfacedetection means for detecting a reflective surface; focus control meansfor performing focus control to the reflective surface such that adistance between a focal point of an optical beam applied to the opticaldisc and the reflective surface is within a predetermined error limit;shift means for shifting the position of the focal point in a directionperpendicular to the optical disc; and control means for controlling thefocus control means and the shift means, wherein the control meanscontrols the shift means such that the focal point of the optical beamshifts toward the protective layer until a surface of the protectivelayer is detected by the reflective surface detection means, the controlmeans controls the focus control means to perform focus control to thesurface of the protective layer when the surface of the protective layeris detected, the control means controls the shift means to release thefocus control to the surface of the protective layer and shifts thefocal point of the optical beam toward the information recording layeruntil a surface of the information recording layer is detected by thereflective surface detection means, and the control means controls thefocus control means to perform focus control to the surface of theinformation recording layer when the surface of the informationrecording layer is detected.
 2. An optical disc unit according to claim1, wherein a feedback gain of the focus control to the surface of theprotective layer and a feedback gain of the focus control to the surfaceof the information recording layer are set such that the product of thefeedback gain of the focus control to the surface of the protectivelayer and a reflectance of the surface of the protective layer is equalto the product of the feedback gain of the focus control to the surfaceof the information recording layer and a reflectance of the surface ofthe information recording layer.
 3. An optical disc unit according toclaim 1, wherein: information indicating the reflectance of theinformation recording layer is formed beforehand on the surface of theprotective layer, the control means reads the information from thesurface of the protective layer while the focus control to the surfaceof the protective layer is performed, and sets the feed back gain of thefocus control to the surface of the information recording layer based onthe information.
 4. An optical disc unit according to claim 1, whereinthe reflectance of the surface of the protective layer is 3% to 5%. 5.An optical disc unit for an optical disc having one or more informationsurfaces having a plurality of tracks formed thereon, comprising:tracking error detection means for detecting a misalignment between anoptical beam applied to the optical disc and one of the plurality of thetracks corresponding thereto, and outputting a tracking error signalindicating the misalignment; amplification detection means for detectingamplification of the tracking error signal; focus control means forperforming focus control such that a distance between a focal point ofthe optical beam and the information surface is within a predeterminederror limit; shift means for shifting the position of the focal point ofthe optical beam toward the optical disc; and control means forcontrolling the focus control means and the shift control means, whereinthe control means controls the shift means such that the focal point ofthe optical beam is shifted in a direction traversing tracks formed onthe information surface of the optical disc and approaches the opticaldisc with an operation of the focus control means stopped; and thecontrol means allows the focus control means to start the operation onlywhen the amplification of the tracking error signal is detected tobecome a predetermined value or higher by the amplification detectionmeans.
 6. An optical disc unit according to claim 5, wherein each of theplurality of the tracks formed on the information surface is wavy.
 7. Anoptical disc unit according to claim 5, further comprising zero-crossdetection means for detecting that a focus error signal indicating amisalignment between the focal point of the optical beam and theinformation surface crosses zero, wherein the control means starts anoperation of the focus control means when the amplification of thetracking error signal is detected to become the predetermined value orhigher by the amplification detection means and the focus error signalis detected to cross zero by the zero-cross detection means.
 8. Anoptical disc unit according to claim 5, further comprising a band-passfilter, wherein the tracking error signal is supplied to theamplification detection means via the band-pass filter.
 9. An opticaldisc unit according to claim 5, wherein the control means controlsrotations of the optical disc such that the number of rotations of theoptical disc when the amplification of the tracking error signal isdetected by the amplification detection means is smaller than the numberof rotations of the optical disc when information recorded on theinformation surface of the optical disc is being reproduced.
 10. Anoptical disc unit according to claim 5, wherein the control meanscontrols strength of the optical beam such that strength of the opticalbeam when the amplification of the tracking error signal is detected bythe amplification detection means is smaller than strength of theoptical beam when information recorded on the information surface of theoptical disc is being reproduced.
 11. An optical disc unit according toclaim 5, wherein the control means performs the focus control withrotations of the optical disc stopped and controls the rotations of theoptical disc such that the optical disc starts to rotate after thedistance between the focal point of the optical beam and the informationsurface is detected to be within the predetermined error limit.
 12. Anoptical disc unit for an optical disc having one or more informationsurfaces, comprising: focus error detection means for outputting a focuserror signal indicating a misalignment between a focal point of anoptical beam applied to the optical disc and a predetermined surface;shift means for shifting the position of the focal point of the opticalbeam in a direction perpendicular to the optical disc; focus controlmeans for performing focus control to the predetermined surface suchthat a distance between the focal point of the optical beam and thepredetermined surface is within a predetermined error limit bycontrolling the shift means based on the focus error signal; zero-crossdetection means for detecting that the focus error signal crosses zero;and control means for controlling the focus control means and the shiftmeans, wherein the control means controls the shift means such that thefocal point of the optical beam shifts in a first direction toward asurface of the optical disc until the focus error signal is detected tocross zero for the first time by the zero-cross detection means, thecontrol means controls the shift means such that, when the focus errorsignal is detected to cross zero for the first time, the focal point ofthe optical beam further shifts in the first direction by apredetermined distance which is larger than a distance between thesurface of the optical disc and the information surface, the controlmeans controls the shift means such that, until the focal point of theoptical beam has been further shifted in the first direction by thepredetermined distance and when the focus error signal is detected tocross zero for the second time by the zero-cross detection means, thefocal point of the optical beam is shifted toward the informationsurface in a second direction opposite to the first direction, and thecontrol means controls the focus control means to perform the focuscontrol to the information surface when the focus error signal isdetected to cross zero for the second time.
 13. An optical disc unitaccording to claim 12, wherein the control means performs the focuscontrol with rotations of the optical disc stopped and controls therotations of the optical disc such that the optical disc starts torotate after the distance between the focal point of the optical beamand the information surface is detected to be within the predeterminederror limit.
 14. An optical disc unit for an optical disc having one ormore information surfaces, comprising: focus error detection means foroutputting a focus error signal indicating a misalignment between afocal point of an optical beam applied to the optical disc and apredetermined surface; shift means for shifting the position of thefocal point of the optical beam in a direction perpendicular to theoptical disc; focus control means for performing focus control to thepredetermined surface such that the distance between the focal point ofthe optical beam and the predetermined surface is within a predeterminederror limit by controlling the shift means based on the focus errorsignal; zero-cross detection means for detecting that the focus errorsignal crosses zero; and control means for controlling the focus controlmeans and the shift means, wherein the control means controls the shiftmeans such that the focal point of the optical beam shifts toward asurface of the optical disc until the focus error signal is detected tocross zero for the first time by the zero-cross detection means, thecontrol means controls the focus control means to perform focus controlto the surface of the optical disc when the focus error signal isdetected to cross zero for the first time, the control means storesdisplacement information indicating displacement of the shift means inaccordance with a rotation angle of the optical disc in storage meanswhile the focus control to the surface of the optical disc is performed,the control means controls the shift means such that the focal point ofthe optical beam shifts toward the information surface based on thedisplacement information stored in the storage means with an operationof the focus control means stopped until the focus error signal isdetected to cross zero for the second time by the zero-cross detectionmeans, and the control means controls the focus control means to performthe focus control to the information surface when the focus error signalis detected to cross zero for the second time.
 15. An optical disc unitaccording to claim 14, wherein the focus control means controls phasecompensation such that a band in which a phase leads is wider, comparedto when information recorded on the optical disc is being reproduced,for a predetermined period after the focus control means has started theoperation.
 16. An optical disc unit according to claim 14, wherein thefocus control means sets a gain such that the gain is smaller, comparedto when information recorded on the optical disc is being reproduced,for a predetermined period after the focus control means has started theoperation.
 17. An optical disc unit for an optical disc having aplurality of information surfaces, comprising: photodetection means fordetecting light reflected off the optical disc when an optical beam isapplied to a predetermined surface among the plurality of informationsurfaces; focus error detection means for outputting a focus errorsignal indicating a misalignment between a focal point of the opticalbeam and the predetermined information surface based on an output fromthe photodetection means; total internal reflection amount detectionmeans for detecting an amount of total internal reflection off theoptical disc based on the output from the photodetection means; andnormalization means for generating a normalized focus error signal bydividing the focus error signal by a value obtained by subtracting asignal value corresponding to a reflection amount reflected offinformation surfaces other than the predetermined information surface ofthe optical disc from the output of the total internal reflection amountdetection means.
 18. An optical disc unit according to claim 17, furthercomprising: shift means for shifting the position of the focal point ofthe optical beam in a direction perpendicular to the optical disc; focuscontrol means for performing focus control such that a distance betweenthe focal point of the optical beam and the predetermined informationsurface is within a predetermined error limit by controlling the shiftmeans based on the normalized focus error signal; and focus gainmeasurement means for measuring a gain of a system of the focus control,wherein the signal value varies depending on an output from the focusgain measurement means.
 19. An optical disc unit according to claim 17,further comprising shift means for shifting the position of the focalpoint of the optical beam in a direction perpendicular to the opticaldisc, wherein the signal value varies such that amplification of thenormalized focus error signal is a constant value when the shift meansis driven such that the focal point of the optical beam passes throughthe predetermined information surface of the optical disc.
 20. Anoptical disc unit according to claim 17, wherein the signal value variesdepending on each of the plurality of the information surfaces.
 21. Anoptical disc unit according to claim 20, further comprising stray lightdetection means for detecting light reflected off information surfacesother than the predetermined information surface of the optical disc onwhich the focal point of the optical beam is located, wherein the signalvalue varies based on an output from the stray light detection means.22. An optical disc unit according to claim 17, further comprising:shift means for shifting the position of the focal point of the opticalbeam in a direction perpendicular to the optical disc; and control meansfor controlling the shift means based on the normalized focus errorsignal so as to control the shift means to shift the focal point of theoptical beam to information surfaces other than the predeterminedinformation surface of the optical disc.
 23. An optical disc unitaccording to claim 22, wherein: the photodetection means furtherincludes optical beam splitting means for splitting light reflected offthe optical disc into light of an inner region near an optical axis andlight of an outer region far from the optical axis; the focus errordetection means includes inner focus error detection means for detectinga misalignment between the focal point of the optical beam and thepredetermined information surface of the optical disc based on the lightof the inner region, and outer focus error detection means for detectingthe misalignment between the focal point of the optical beam and thepredetermined information surface of the optical disc based on the lightof the outer region; and the control means controls the shift meansbased on at least one of an output from the inner focus error detectionmeans and an output from the outer focus error detection means so as tocontrol the shift means to shift the focal point of the optical means toinformation surfaces other than the predetermined information surface ofthe optical disc.